Octa-coordinated alkaline earth metal–dinitrogen complexes M(N2)8 (M=Ca, Sr, Ba)

Wang, Qian; Pan, Sudip; Lei, Shujun; Jin, Jiaye; Deng, Guohai; Wang, Guanjun; Zhao, Lili; Zhou, Mingfei; Frenking, Gernot

doi:10.1038/s41467-019-11323-5

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Published: 29 July 2019

Octa-coordinated alkaline earth metal–dinitrogen complexes M(N₂)₈ (M=Ca, Sr, Ba)

Nature Communications volume 10, Article number: 3375 (2019) Cite this article

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Abstract

We report the isolation and spectroscopic identification of the eight-coordinated alkaline earth metal–dinitrogen complexes M(N₂)₈ (M=Ca, Sr, Ba) possessing cubic (O_h) symmetry in a low-temperature neon matrix. The analysis of the electronic structure reveals that the metal-N₂ bonds are mainly due to [M(d_π)]→(N₂)₈ π backdonation, which explains the observed large red-shift in N-N stretching frequencies. The adducts M(N₂)₈ have a triplet (³A_1g) electronic ground state and exhibit typical bonding features of transition metal complexes obeying the 18-electron rule. We also report the isolation and bonding analysis of the charged dinitrogen complexes [M(N₂)₈]⁺ (M=Ca, Sr).

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Introduction

The chemical conversion of dinitrogen to commercially useful commodities is one of the most important reactions in chemical industry, which has long been the subject of extensive research¹. Activating the strong triple bond in N₂ is a challenge for inventive chemists, who tried in the past various transition metals for binding the molecule as ligand in molecular complexes. Dinitrogen is a poorly binding donor due to its comparatively weak donor–acceptor interactions in transition metal complexes^1,2. Metal–ligand bonds of N₂ are usually much weaker than those of isoelectronic CO. Braunschweig and coworkers recently reported that boron compounds may also be utilized for fixation of dinitrogen^3,4.

Very recently, we reported the isolation and spectroscopic identification of the eight-coordinated carbonyl complexes M(CO)₈ of the alkaline earth atoms M=Ca, Sr, Ba possessing cubic (O_h) symmetry in a low-temperature matrix⁵. The analysis of the electronic structure showed that the classical main-group metals M bind the CO ligands via donor–acceptor interactions through their (n)d atomic orbitals thus mimicking transition metals. Now we found that the alkaline earth atoms may also bind N₂ in octa-coordinated complexes M(N₂)₈ (M=Ca, Sr, Ba) whereby the loss of one N₂ ligand has surprisingly only a slightly smaller bond dissociation energy than the dissociation of one carbonyl ligand from M(CO)₈. Homoleptic dinitrogen complexes of genuine transition metals, such as tetra-coordinated Ni(N₂)₄, hexa-coordinated vanadium, and chromium complexes M(N₂)₆, have been synthesized and spectroscopically characterized in low-temperature matrices^6,7,8,9. Mass spectrometric and infrared photodissociation spectroscopic investigations in the gas phase revealed that the Ti⁺, V⁺, and Nb⁺ cations form six-coordinated dinitrogen complexes^10,11,12, while the Y⁺, La⁺, and Ce⁺ cations gave the octa-coordinated dinitrogen complexes¹³. There is only one theoretical report on neutral octa-coordinated dinitrogen complexes by Kovacs, who calculated the lanthanum species La(N₂)_n (n = 1–8)¹⁴. The finding that heavier alkaline earth atoms may bind eight N₂ ligands in forming the neutral M(N₂)₈ complexes is unprecedented.

Results

Experimental studies

Figure 1 shows the spectra in the terminal N–N stretching frequency region from the experiment using a barium target and a 0.5% N₂/Ne sample. The spectra were recorded after (a) 30 min of sample deposition at 4 K, (b) after annealing at 12 K, (c) after 15 min of visible light irradiation, and (d) after 15 min of UV-visible light irradiation. Only two bands at 2118.0 and 2237.6 cm⁻¹ were observed. The 2237.6 cm⁻¹ band is metal independent and can be assigned to the antisymmetric stretching vibration of the linear N₄⁺ cation based on the literature data¹⁵. The 2118.0 cm⁻¹ band increases tremendously under UV-visible light irradiation. The weak band at 2141 cm⁻¹ is due to trace of CO impurity absorption.

Fig. 1

Full size table

Figure Full size table

The data in Table Full size table

The attractive metal–ligand interactions ΔE_int of M(N₂)₈ come mainly from the covalent (orbital) term ΔE_orb. The breakdown of the latter term into pairwise orbital interactions ΔE_orb(1)–ΔE_orb(5) shows that the covalent bonding is dominated by the [M(d)]→(N₂)₈ π backdonation ΔE_orb(1), which contributes 70–85% to the total orbital interactions. The rather diffuse (n−1)d orbitals give the alkaline earth elements Ca–Ba an unusual reactivity with a remarkable donor ability. This was previously noted in our study of the carbonyl cation complexes [Ba(CO)]⁺ where the metal cation Ba⁺ is a donor for the neutral CO ligand^{Full size table}

Methods

The alkaline earth metal–dinitrogen complexes were prepared via the reactions of pulsed laser-evaporated alkaline earth metal atoms and dinitrogen molecules in solid neon. The product species were detected by infrared absorption spectroscopy employing a Bruker Vertex 80 V spectrometer at 0.5 cm⁻¹ resolution using a liquid-nitrogen cooled mercury cadmium telluride (MCT) detector. The experiments were carried out with a wide range of dinitrogen concentrations (from 0.02 to 2% relative to Ne on the basis of volume). No obvious product absorptions were observed in the experiments with very low N₂ concentrations (<0.05%). Further details have been described before²⁸.

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Acknowledgements

The experimental work was supported by the National Natural Science Foundation of China (grant numbers 21688102 and 21433005). L.Z. and G.F. acknowledge financial support from Nanjing Tech University (grant numbers 39837123 and 39837132) and SICAM Fellowship from Jiangsu National Synergetic Innovation Center for Advanced Materials, Natural Science Foundation of Jiangsu Province for Youth (grant number BK20170964), National Natural Science Foundation of China (grant number 21703099). S.P. thanks Nanjing Tech University for a postdoctoral fellowship and the high performance center of Nanjing Tech University for the computational resources. G.F. and S.P. are grateful to the Deutsche Forschungsgemeinschaft for financial support.

Author information

These authors contributed equally: Qian Wang, Sudip Pan.

Authors and Affiliations

Department of Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai, 200433, China
Qian Wang, Shujun Lei, Jiaye Jin, Guohai Deng, Guanjun Wang & Mingfei Zhou
Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing, 211816, China
Sudip Pan, Lili Zhao & Gernot Frenking
Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Strasse 4, D-35043, Marburg, Germany
Gernot Frenking

Authors

Qian Wang
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Sudip Pan
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Shujun Lei
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Jiaye Jin
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Guohai Deng
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Guanjun Wang
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Lili Zhao
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Mingfei Zhou
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Gernot Frenking
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Contributions

S.P. carried out the quantum chemical calculations. L.Z. and G.F. analyzed the theoretical data and wrote the theoretical part of the paper. Q.W., G.D., and G.W. did the matrix isolation experiments, S.L. and J.J. did the gas phase infrared photodissociation experiments. M.Z. analyzed the experimental data and wrote the experimental part of the paper. M.Z. and G.F. supervised the experimental and theoretical work. Q.W. and S.P. contributed equally to this work.

Corresponding authors

Correspondence to Mingfei Zhou or Gernot Frenking.

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Wang, Q., Pan, S., Lei, S. et al. Octa-coordinated alkaline earth metal–dinitrogen complexes M(N₂)₈ (M=Ca, Sr, Ba). Nat Commun 10, 3375 (2019). https://doi.org/10.1038/s41467-019-11323-5

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Received: 29 April 2019
Accepted: 09 July 2019
Published: 29 July 2019
DOI: https://doi.org/10.1038/s41467-019-11323-5

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