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Highly Coordinated Heteronuclear Calcium–Iron Carbonyl Cation Complexes [CaFe(CO) n ] + ( n =5–12) with d−d Bonding

Xiaoyang Jin,Yuna Bai,Yangyu Zhou,Guanjun Wang,Lili Zhao,Mingfei Zhou,G. Frenking

Published 2021 in Angewandte Chemie

ABSTRACT

Heteronuclear calcium–iron carbonyl cation complexes in the form of [CaFe(CO)n] + (n = 5–12) are produced in the gas phase. Infrared photodissociation spectroscopy in conjunction with quantum chemical calculations confirm that the n = 10 complex is the coordination saturated ion where a Fe(CO)4 fragment is bonded with a Ca(CO)6 fragment through two side-on bridging carbonyl ligands. Bonding analysis indicates that it is best described by the bonding interactions between a [Ca(CO)6] 2+ dication and an [Fe(CO)4] anion forming a Fe!Ca d d dative bond in the [(CO)6Ca–Fe(CO)4] + structure, which enriches the pool of experimentally observed complexes of calcium that mimic transition metal compounds. The molecule is the first example of a heteronuclear carbonyl complex featuring a d d bond between calcium and a transition metal. The alkaline earth elements are among the most electropositive elements in the periodic table and they are usually classified as main-group elements that belong to the s-block atoms. They readily lose the outermost valence s electrons to form ionic compounds. The coordination chemistry of alkaline earth metal ions was thus considered to be relatively simple and uninteresting for quite a long time. The metal– ligand interactions are expected to be predominantly electrostatic with negligible orbital interactions, and are mostly governed by Coulomb interactions and steric factors. 2] However, recent investigations indicate that the alkaline earth elements have a much richer coordination chemistry than hitherto thought. Many alkaline earth metal coordination compounds show unusual geometric and electronic structures with covalent bonding character. In particular, the zero and monovalent heavier alkaline earth metals show an unexpected bonding situation that mimic transition metals. It was found that the heavier group-2 atoms form octa-coordinated carbonyl and dinitrogen complexes M(CO)8 and M(N2)8 (M = Ca, Sr, Ba) as well as tribenzene complexes M(Bz)3 (M = Sr, Ba) in solid neon matrix. [9–11] The bonding situation in these complexes is similar to transition metal complexes following the 18 electron rule with the metal d orbitals playing a crucial role in metal–ligand bonding. The octa-coordinated carbonyl and dinitrogen complexes of monovalent metal ions were also formed in the gas phase, which show similar bonding as the neutral complexes. Computational investigations indicate that d orbitals also contribute significantly to the bonding of divalent heavier alkaline earth metal complexes. A recent theoretical analysis of the chemical bonds in alkaline earth oxides, imides and dihydrides as well as a calcium complex suggests that the valence orbitals of Ca, Sr, Ba comprise the (n)s and (n 1)d AOs, the latter being much more important than the former, which is a characteristic feature of the transition metals. Here we report the generation and spectroscopic characterization of heteronuclear calcium–iron carbonyl cation complexes in the gas phase. Infrared spectroscopy in conjunction with theoretical calculations confirm that the coordinatively saturated [CaFe(CO)10] + cation possesses a [Ca(CO)6] [Fe(CO)4] structure involving significant Fe Ca d d dative bonding supported by two side-on bridging carbonyl ligands. The heteronuclear calcium–iron carbonyl cation complexes are prepared by a pulsed laser vaporization/supersonic expansion ion source in the gas phase, and are detected by mass-selected infrared photodissociation spectroscopy in the carbonyl stretching vibrational frequency region as described in detail previously. The mass spectrum of ion complexes produced by pulsed laser vaporization of a pure calcium metal target in an expansion of helium seeded with carbon monoxide with traces of Fe(CO)5 impurity is shown in Figure 1. The spectrum is dominated by a progression of mass peaks that can be assigned to the heteronuclear calcium– iron carbonyl cation complexes in the form of [CaFe(CO)n] + (n = 4–12). The [Ca(CO)n] + (n = 0–5) peaks are also presented. The [CaFe(CO)10] + cation is always the most intense peak among this series of cations at different experimental [*] X. Jin, Y. Zhou, G. Wang, M. Zhou Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University Shanghai 200438 (China) E-mail: mfzhou@fudan.edu.cn Y. Bai, L. Zhao, G. Frenking Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, Nanjing Tech University Nanjing 211816 (China)

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