Electronic coupling of two [Cp*ClM]+/[Cp*M] reaction centers via π conjugated bridging ligands: similarities and differences between rhodium and iridium analogues

doi:10.1016/S0022-328X(98)01027-4

Journal of Organometallic Chemistry

Volume 582, Issue 2, 20 June 1999, Pages 153-159

https://doi.org/10.1016/S0022-328X(98)01027-4 Get rights and content

Abstract

The dinuclear complexes [Cp*ClM(μ-L)MClCp*](PF₆)₂, M=Rh or Ir, L=3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) or 2,5-bis(phenyliminoethyl)pyrazine (bpip), are reduced in several chemically reversible steps by up to six electrons to the species [Cp*M(μ-L)MCp*]ⁿ⁻. UV–vis/NIR spectroelectrochemistry and EPR of the paramagnetic states were used to identify the various intermediates. The complexes clearly show a reversible, ligand-centered one-electron reduction (E) preceding the first chloride-dissociative metal reduction step (EC). Metal–metal interaction via the bridging π acceptor ligand L causes a splitting of 310–710 mV between the potentials for the two Cl⁻-dissociative steps. The second chloride release occurs in EC+E fashion for L=bpip but in a two-electron process for L=bptz. The M^IIM^I mixed-valent species [Cp*M(μ-L)MCp*]⁺ could be identified via long-wavelength bands from intervalence charge transfer (IVCT) transitions. All complexes containing at least one chloride-free Cp*M group display intense long-wavelength absorption bands. The iridium complexes are distinguished by more negative potentials of the [Cp*Ir]-containing forms, by slower formation of the M₂^I,II mixed-valent intermediate, by larger g anisotropy of the paramagnetic forms, and by triplet absorption features in the UV–vis electronic spectra.

Introduction

The electronic coupling of one-electron redox processes occurring at individual metal centers as mediated by bridging ions or molecules has played an enormous role in the understanding of intra- and intermolecular electron transfer [1]. Typical examples for compounds with equivalent one-electron redox sites are the ligand-bridged dinuclear ammineruthenium(III,II) complexes where the K_c value varies from less than 10¹ to 10¹⁵ [2], [3]. $K_{c} =10^{ΔE/59mV} =[M^{(n−1)}]^{2} /[M^{n}][M^{(n−2)}]$ $M^{n} + M^{(n−2)} ⇌2M^{(n−1)}$ The latter value was observed for a compound involving the 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) as conjugated bridging ligand with low lying π acceptor orbitals and large π* MO coefficients at the coordinating tetrazine nitrogen centers [3], [4].

For chemical reactions proper, however, the electronic coupling of processes including a chemical step would be more interesting. Among the most common and well understood of such composite processes are the cyclical EC, EEC or ECE mechanisms where a chemical reaction (C) following electron transfer (E) produces a new species with different electrode potentials which eventually reverts in a second chemical step to the starting compound [5].

We are now reporting an investigation of such an electronic coupling of reaction centers (instead of mere electron transfer centers) which involves two equivalent pentamethylcyclopentadienyliridium centers starting from complex ions [Cp*ClIr(μ-L)IrClCp*]²⁺, L=3,6-bis(2-pyridyl)-1,2,4,5-tetrazine or 2,5-bis(phenyliminoethyl)pyrazine (bpip), Cp*=η⁵-C₅Me₅. Whereas the bptz bridging ligand is distinguished by its capability to effect strong metal–metal coupling and stabilized paramagnetic species [3], [4], [6], bpip is a bis(α-diimine) ligand which acts through two different chelate donors, one imine and one azine nitrogen atom per metal center [7], [8]. Thus, bpip may be viewed as a centrosymmetrically replicated 2-pyridinecarbaldimine (pyca) system [9], [10].

A study of corresponding dirhodium analogues has been described recently [8] and those results will be used here for comparison. When bound to an α-diimine ligand such as 2,2′-bipyridine single Cp*ClM⁺ fragments undergo an ECE process whereby the initial electron acquisition is followed by a rapid dissociation of chloride as the chemical step [11]. The resulting, catalytically active and deeply colored neutral Rh(I) or Ir(I) compounds are reoxidized at a significantly less negative potential upon which they pick up the additional ligand [12], [13], [14]. This last step is rather slow as obvious from cyclic voltammetry; the introduction of bulky substituents can considerably delay this process [15], [16] and allow for the isolation and structural characterization of the species involved [16].

Compounds of the general type [Cp*ClM(α-diimine)]⁺, M=Rh or Ir, have been used for purposes of catalytic hydride transfer, e.g. to protons [11], [14], [17] or NAD⁺[18]; expectedly, the Rh analogues showed the higher activity.

Section snippets

Results and discussion

Complexes [Cp*ClIr(μ-L)IrClCp*](PF₆)₂ were obtained from the ligands L [4], [7] and [Cp*ClIr(μ-Cl)]₂ [19]. The appearance of close lying pairs of NMR resonance signals for the bpip compounds and for the rhodium complex of bptz suggest the formation of cis and trans isomers with regard to the positions of Cp* and Cl relative to the π plane of L [8], [20]. As has been shown before [8], [20], this isomerism has only marginal effects on electrochemistry (Table 1) and absorption spectra (Table 2);

Experimental

Instrumentation and spectroelectrochemical techniques were described previously ([13b]) as were the compounds [Cp*ClRh(μ-L)RhClCp*](PF₆)₂ [8].

Compounds [Cp*ClIr(μ-L)IrClCp*](PF₆)₂ were obtained from reacting [Cp*Cl₂Ir]₂ [19] with two equivalents of AgPF₆ in acetone and addition of one equivalent of the bridging ligand L [4], [7] to the filtrate after removal of AgCl through celite. After 1 h the deeply colored solutions were reduced in volume, filtered, and the filtrate treated with excess Bu₄

Acknowledgements

This work was supported by Deutsche Forschungsgemeinschaft (DFG, SFB 270). We thank Degussa AG for a loan of IrCl₃.

References (26)

M. Ladwig et al.
J. Organomet. Chem.
(1991)
F. Baumann et al.
Inorg. Chem. Commun.
(1998)
A. Klein et al.
Inorg. Chim. Acta
(1997)
W. Kaim et al.
Inorg. Chim. Acta
(1993)
(a) J.R. Bolton, N. Mataga, G. McLendon (Eds.), Electron Transfer in Inorganic, Organic and Biological Systems, Adv....
C. Creutz
Prog. Inorg. Chem.
(1983)
J. Poppe et al.
Inorg. Chem.
(1993)
W. Kaim et al.
Inorg. Chem.
(1987)
D. Astruc
Electron Transfer and Radical Processes in Transition Metal Chemistry
(1995)
A. Klein et al.
Organometallics
(1998)

(a) A. Klein, V. Kasack, R. Reinhardt, T. Sixt, T. Scheiring, S. Zalis, J. Fiedler, W. Kaim, J. Chem. Soc. Dalton...

W. Kaim, R. Reinhardt, J. Fiedler, Angew. Chem. 109 (1997) 2600. Angew. Chem. Int. Ed. Engl. 36 (1997)...

(a) H. Brunner, W.A. Herrmann, Chem. Ber. 105 (1972) 770. (b) L.H. Polm, C.J. Elsevier, G. van Koten, J.M. Ernsting,...

Cited by (33)

Organometallic complexes of functionalized chelating azines
2020, Advances in Heterocyclic Chemistry
Citation Excerpt :
For the rhenium(I) case, the mononuclear N,N-chelate can also be prepared. Dinuclear rhodium(III) and iridium(III) bis-chelates can be prepared in a similar fashion (1997AGE2493, 1999JOM(582)153, 2000IC2516). Rhenium(I) mononuclear complex reveals ligating properties and affords heterodinuclear rhenium(I)–rhodium(III) and rhenium(I)–iridium(III) complexes with bridging ligand (Eq. 129) (2004D3727, 2004ZAAC1883, 2008JMS133).
Organometallic compounds of the chelating azines containing carbonyl, hydroxyl, thiol, selenol, amino, imino, phosphino, thienyl, indolyl, carbazolyl, pyrazolyl, imidazolyl, 1,2,3-triazolyl, and N-heterocyclic carbene functionalities are reviewed. Synthesis and coordination modes, the role of the discussed compounds in catalysis, materials chemistry, photochemistry, and microbiology are highlighted.
Cp∗ versus Bis-carbonyl iridium precursors as CH oxidation precatalysts
2017, Organometallics
We previously reported a dimeric Ir^IV-oxo species as the active water oxidation catalyst formed from a Cp*Ir(pyalc)Cl {pyalc = 2-(2′-pyridyl)-2-propanoate} precursor, where the Cp* is lost to oxidative degradation during catalyst activation; this system can also oxidize unactivated CH bonds. We now show that the same Cp*Ir(pyalc)Cl precursor leads to two distinct active catalysts for CH oxidation. In the presence of external CH substrate, the Cp* remains ligated to the Ir center during catalysis; the active specieslikely a high-valent Cp*Ir(pyalc) specieswill oxidize the substrate instead of its own Cp*. If there is no external CH substrate in the reaction mixture, the Cp* will be oxidized and lost, and the active species is then an iridium-μ-oxo dimer. Additionally, the recently reported Ir(CO)₂(pyalc) water oxidation precatalyst is now found to be an efficient, stereoretentive CH oxidation precursor. We compare the reactivity of Ir(CO)₂(pyalc) and Cp*Ir(pyalc)Cl precursors and show that both can lose their placeholder ligands, CO or Cp*, to form substantially similar dimeric Ir^IV-oxo catalyst resting states. The more efficient activation of the bis-carbonyl precursor makes it less inhibited by obligatory byproducts formed from Cp* degradation, and therefore the dicarbonyl is our preferred precatalyst for oxidation catalysis.
Mixed valency of a 5d element: The osmium example
2013, Coordination Chemistry Reviews
Excited state dynamics of 3,6-diaryl-1,2,4,5-tetrazines. Experimental and theoretical studies
2012, Journal of Photochemistry and Photobiology A: Chemistry
Citation Excerpt :
As the synthesis of 3,6-subsituted tetrazines is rather straightforward [3], they can readily be implemented in various functional molecular systems, such as polymers in which tetrazine derivatives are coupled to electron-rich thiophene groups [4–7]. Tetrazine derivatives have also been frequently used as bridging ligands in metal complexes in which they facilitate metal-metal interactions [8,9] or in switchable metal complexes [10]. Tetrazine derivatives have a low energy absorption band in the visible region, which is weakly influenced by the nature of the substituents [5,11].
The photophysical behavior was studied of three symmetrical 1,2,4,5-tetrazine derivatives substituted with two aromatic substituents (phenyl: DPT; p-methoxyphenyl: DAT; thiophen-2-yl: DTT). The UV–visible absorption spectra of these compounds in cyclohexane and acetonitrile show two absorption maxima at 500–550 nm and 290–330 nm, as well as a shoulder at lower energies on the latter absorption band. The electronic transitions were assigned on the basis of TD-DFT calculations. In contrast with some other tetrazine derivatives, these compounds exhibit only weak fluorescence (Φ_f ≈ 10⁻⁴ to 10⁻³) from the S₁ (nπ*) state. When the molecules are excited to a higher energy ππ* state, fluorescence from the nth excited state (n = 6 for DPT, n = 5 for DAT and n = 4 for DTT) is detected. Time-correlated single photon counting (TC-SPC) and femtosecond transient absorption (fs-TA) measurements showed that internal conversion from the S_n state to the S₁ state is unusually slow, of the order of 30 ps for DTT and 20 ps for DAT.
Concepts for metal complex chromophores absorbing in the near infrared
2011, Coordination Chemistry Reviews
Citation Excerpt :
The use of the even better π accepting 2,3,5,6-tetrafluoro-TCNQ ligand in XXII, n = 7, reveals a slightly higher metal contribution to the singly occupied MO and an even further shifted NIR absorption at λmax = 1822 nm (Fig. 9) [31b]. While the spectra of {(μ4-TCNQ)[Ru(NH3)5]4}7+ (XXI) and of the tetrafluoro-substituted analogue (XXII, n = 7) are identified [83] as shifted absorptions known from the free ligand [28], the original non-reduced (8+) ions also have intense near IR absorptivity (Fig. 9) which results from weakly coupled mixed-valent (Ru2.5)2 terminal groups (dimer-of-dimers model) [31,83]. Near infrared absorption of oligonuclear compounds is not only expected with non-innocent ligands bridging single metal centers but also when metal cluster entities with close-lying frontier orbitals are electronically coupled [80].
The near-infrared (750–2500 nm) region of the electromagnetic spectrum has received enhanced attention recently from the areas of atmospheric and analytical chemistry, from communications technology, and from the side of medical applications. Coordination compounds absorbing strongly in this region can do so because of extended ligand π systems, mixed valency, or the presence of radical ion ligands. The background of such low-energy absorptivity is provided, together with examples from the inorganic and organometallic coordination chemistry of Mo, Mn, Fe, Ru, Os, Rh, Ir, Pt and Cu.
The 1,4-diazabutadiene/1,2-enediamido non-innocent ligand system in the formation of iridaheteroaromatic compounds: Spectroelectrochemistry and electronic structure
2010, Journal of Organometallic Chemistry
Oxidation of the unambiguously characterized iridium(III)-enediamido complex IrCp∗(RNCHCHNR), R = 2,6-dimethylphenyl, with six cyclically arranged π electrons was investigated using EPR and UV–vis spectroelectrochemistry. In contrast to a corresponding iminocatecholato system reported recently by Rauchfuss et al. [27] the two one-electron oxidation steps are not completely reversible, depending on the solvent. We attribute the electrochemical behavior observed in weakly coordinating dichloromethane and propylene carbonate solvents to an enhanced propensity for ligand addition: while neutral IrCp∗(RNCHCHNR) remains coordinatively unsaturated with the strongly σ and π donating enediamido(2−) ligand mitigating the electron deficit, the oxidation to the less donating radical anion ligand (RNCHCHNR)^- or even to neutral 1,4-diazabutadiene is considered to induce a strong tendency for quasi-hexacoordination in [IrCp∗(L)(RNCHCHNR)], L = solvent, halide, substrate. The presence of excess chloride thus leads to formation of the precursor complex ion [IrCp∗ClRNCHCHNR)]⁺ after oxidation. EPR spectroscopy of the one-electron oxidation intermediate [IrCp∗(RNCHCHNR)]⁺ was successful only for a frozen solution in propylene carbonate/0.1 M Bu₄NPF₆ which revealed an axial signal with sizeable g anisotropy. DFT calculation results for [IrCp∗(RNCHCHNR)]ⁿ, n = 0, +, 2+ confirm the available structural and spectroelectrochemical data (UV–vis, EPR).

View all citing articles on Scopus

^☆: Dedicated to Professor Peter Jutzi on the occasion of his 60th birthday.

View full text

Electronic coupling of two [Cp*ClM]+/[Cp*M] reaction centers via π conjugated bridging ligands: similarities and differences between rhodium and iridium analogues☆

Abstract

Introduction

Section snippets

Results and discussion

Experimental

Acknowledgements

J. Organomet. Chem.

Inorg. Chem. Commun.

Inorg. Chim. Acta

Inorg. Chim. Acta

Prog. Inorg. Chem.

Inorg. Chem.

Inorg. Chem.

Electron Transfer and Radical Processes in Transition Metal Chemistry

Organometallics

Electronic coupling of two [CpClM]⁺/[CpM] reaction centers via π conjugated bridging ligands: similarities and differences between rhodium and iridium analogues☆