Diradical Character and Second Hyperpolarizability of Alkaline Earth Metal Inverse Sandwich Complexes Involving Cyclopentadienyl and Cyclooctatetraene Ligands

• Published in: European journal of inorganic chemistry 2018-07, Vol.2018 (25), p.2894-2899
• Main Authors: Takamuku, Shota, Nakano, Masayoshi
• Format: Article
• Language: English
• Subjects: Alkaline earth metals; Calcium; Configuration interaction; Coordination compounds; Earth; Inorganic chemistry; Ligands; Magnesium; Nonlinear optics; Optical properties; Organic chemistry; Organometallic compounds; Quantum chemistry; Radicals; Sandwich complexes
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201800267

Using strong‐correlated quantum chemical calculations, we have revealed the relationship between diradical character y and second hyperpolarizability γ [third‐order nonlinear optical (NLO) property] in alkaline earth metal inverse sandwich complexes, which have Mg or Ca atoms as an alkaline earth metal and a four‐ or eight‐membered ring as an organic ligand (4‐Mg, 4‐Ca, 8‐Mg, and 8‐Ca). Through detailed analysis of diradical character with the valence configuration interaction (VCI) model, we have found that the higher period metal leads to a larger metal–metal distance with larger y value 0.8–0.9, while a larger ring size of organic ligand leads to smaller diradical character values due to the larger transfer integral (smaller distance) between metals. These features are explained by orbital diagrams and VCI analysis. Indeed, it is found that 8‐Ca has the largest γ value, which is 4.94 times as large as that of the system with the smallest γ value (8‐Mg). From these results, we obtain a design guideline for efficient third‐order NLO systems based on alkaline earth metal inverse sandwich complexes: a combination of alkaline earth metal with a higher period number and larger organic antiaromatic (4nπ) ring tends to enhance the γ value due to the larger metal–metal distance with keeping an intermediate y value. The diradical characters of alkaline earth metal inverse sandwich complexes are controlled by the metal species (Ca, Mg) and ring ligands (cyclopentadienyl, cyclooctatetraene). A combination of higher period alkaline earth metal and larger organic antiaromatic (4nπ) ring tends to enhance the second hyperpolarizability due to the intermediate diradical character and large effective diradical distance.