Reactivity of germanones: far removed from ketones – a computational study† A. K. Jissy, Sanjay K. Meena and Ayan Datta * The recently isolated heavy ketone (X 2 E]O; E ¼ Ge) shows interesting reactivity towards simple addition reactions that are unknown for the organic (E]C) ketones. Density Functional Theory (DFT) calculations elucidate the role of the central atom and the substituent effects on the addition of acetone, carbon dioxide and water to formaldehyde, acetone, 2,4-dimethyl-3-pentanone, diphenyl ketone and their heavier (Si, Ge) analogs. NBO analysis reveals a large charge separation for heavier analogs, thereby increasing their reactivity. The activation barriers are reduced by as much as 30.0 to 40.0 kcal mol 1 for the nucleophilic addition to the heavy ketones as (E ¼ Si and Ge) compared to those of an organic ketone. Comparably lower stability of the reactants and higher stability of the transition state lowers the barriers of addition reactions for the heavier ketones. Apart from a smaller barrier, the nucleophilic addition reactions which are unfavorable for ketones are favored by 20–40 kcal mol 1 for their silicon and germanium counterparts. Calculations predict that similar addition reactions must be feasible for silanones (X 2 Si]O) as well. Introduction The reactivity of the carbon–oxygen double bond in ketones occupies a central role in organic structure and stereochemistry. However, the heavier congeners of group 14 – Si, Ge, Sn and Pb – preferentially form single bonds with oxygen. Owing to the larger electronegativity difference between E (E ¼ group 14 atom) and O atoms in the heavier ketones, compared to the C] O bond, the E]O bond in ketones is inherently polarized, resulting in highly reactive double-bonds in the heavier analogs. 1 Hence, the compounds that are well known for carbon are rare for the heavier atoms. 2,3 Organic ketones exist in their monomeric form, whereas the double bond in heavier ketones are prone to forming polymeric chains, such as the –Si–O–Si–O– Si– chains of polysiloxanes and silicones. 4,5 The highly reactive diorganogermanones, R 2 G]O, also polymerize into cyclo- germanones (R 2 GeO) n (n ¼ 2–4). 6,7 The weaker and more polar nature of the E]O double bonds is understood to be due to the less effective sp 2 -hybridization in the heavier E element as compared to organic ketones. 3 While synthetic chemistry has been essentially utilizing a large variety of organic carbonyl derivatives, Eaborn et al. rst detected silanones in the early 1980s. 8,9 Withnall and Andrews provided infrared spectroscopic evidence to prove the existence of H 2 Ge]O by matrix reactions of germane and oxygen atoms. 10 Many theoretical studies on derivatives containing Si]O groups have been reported. 11,12 The reaction coordinate in the unimolecular reaction of silanones has been quantum mechanically investigated. The trans-HSiOH was found to be the absolute minimum, which is only slightly more stable than the cis isomer (0.2 to 0.5 kcal mol 1 ) as well as silanone H 2 Si]O (1.3 kcal mol 1 ). 13 This is in contrast to the potential energy surfaces for CH 2 O, in which formaldehyde is 54.0 kcal mol 1 more stable than trans-HCOH and 59.1 kcal mol 1 more than the cis-HCOH carbene. 2,14,15 Trinquier and So have investigated the structure of germanone and its derivatives using ab initio calculations. 16–18 They found trans-HGeOH to be 1.0 kcal mol 1 more stable than cis-HGeOH and 19.0 kcal mol 1 more stable than the germanones, H 2 Ge]O. Irrespective of the group 14 X 2 E]O systems being postulated as intermediates, the experi- mental structures of stannanone, H 2 Sn]O and plumbanone, H 2 Pb]O were reported a long time ago. These compounds are also prone to oligomerization and addition reactions. 19–21 The doubly bonded germanium species are highly reactive and widely used in organometallic synthesis in various types of reactions including addition to (or insertion into) various s-bonds of organometallic compounds; insertion reactions with ring expansion of small organic rings and cycloaddition reac- tions with 1–2, 1–3, and 1–4 dipolar reagents. 22 The Ge]O bond remained a transient species in chemistry for a long time. In spite of numerous attempts to synthesize it, germanone was metastable at room temperature. 23 Nevertheless, stable conge- ners of double-bond compounds involving heavier group 14, 15, and 16 elements have been synthesized, exploring the kinetic stabilization afforded by bulky substituents. 24–26 Incorporation Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, 700032, West Bengal, India. E-mail: spad@iacs.res.in † Electronic supplementary information (ESI) available: Cartesian coordinates, energies, harmonic frequencies, complete Gaussian 09 reference. See DOI: 10.1039/c3ra44352c Cite this: RSC Adv., 2013, 3, 24321 Received 14th August 2013 Accepted 18th September 2013 DOI: 10.1039/c3ra44352c www.rsc.org/advances This journal is ª The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 24321–24327 | 24321 RSC Advances PAPER