π-Complexes of transition metal tricarbonyls with cyclopolyenes and their boron analogs

Abstract

ul. Chekhova, 344006 Rostov on Don, Russian FederationThe structures and stability of complexes of transition metal tricarbonyls (M = Co, Fe, Mn, Cr) with hydrocarbon and borane basal rings, (C n H n )M(CO) 3 and (B n H 2n )M(CO) 3 , respec tively, as well as internal rotation of the metal tricarbonyl fragments in these systems were studied by the DFT B3LYP/6 311+G(df,p) method. Replacement of hydrocarbon basal frag ments by isoelectronic borane rings leads to strengthening of the interaction between the apical and basal fragments, but does not change the trend in the heights of barriers to internal rotation. In all cases, the metal tricarbonyl fragment stabilizes the nonclassical planar form of the borane rings, whereas the complexes of metal tricarbonyls based on the classical nonplanar borane conformations are less thermodynamically stable.Key words: transition metal tricarbonyls, metalloboranes, stereochemical nonrigidity, quan tum chemical calculations.π Complexes of transition metal tricarbonyls with cy clopolyenes (C n H n )M(CO) 3 have been well studied. 1,2 To date, various 18 electron complexes (C n H n )M(CO) 3 (n = 4-8) with apical groups containing transition metals from Ti to Ni have been synthesized and structurally char acterized. Such compounds possess high thermal stability and are stereochemical nonrigid due to almost free rota tion of the metal tricarbonyl group about the symmetry axis. A salient feature of these systems is the possibility for induced aromaticity to occur. 3 For instance, the interac tion of the Fe(CO) 3 group with the antiaromatic cyclo butadiene fragment in complex 1 stabilizes the square con figuration of the basal ring with equalized carbon-carbon bond lengths. Compound 1 exhibits characteristic aro matic properties; this made it possible to introduce the term "metalloaromaticity". 4,5The first metallaborane synthesized was ferraborane 2 (Scheme 1) in which the cyclobutadiene ring is replaced by borane. 6 This compound is isostructural and isoelec tronic to the hydrocarbon analog 1. Similarly to the cy clobutadiene fragment, which is stable in the rectangular form with alternating bonds (system with equalized bonds is unstable), the planar four membered borane ring B 4 H 8 is also unstable and has a boat structure in the free state.Thus, the Fe(CO) 3 group stabilizes not only the sym metrical form of cyclobutadiene, but also the nido form of isoelectronic borane B 4 H 8 , which does not exist in the free state. 6-8 This leads to a new stable metallaborane 2.At present, investigations of such systems that relate the chemistry of boranes to organometallic chemistry and co ordination chemistry is a topical avenue of experimental and theoretical research, which offers prospects for the discovery of novel compounds with unusual structures and properties. 6-8One can expect that stabilization of other borane rings B n H 2n (n = 3, 5, 6) by the corresponding metal tricarbonyl fragments will lead to a new family of metallaborane sys tems 3-5 isoelectronic and isostructural to the t...

Induced Aromaticity and Electron-Count Rules for Bipyramidal and Sandwich Complexes of s- and d-Metals

“…The 30-electron (sum of the numbers of -electrons of the basal cycle and valence electrons of apical groups) danalogues of lithium complex 12 are represented by stable bipyramidal complexes C 4 H 4 Mn 2 (CO) 6 of C 2v -symmetry 13 and C 4 H 4 V 2 (CO) 8 of D 4h -symmetry 14 (Fig. 1).…”

“…We have previously introduced the notion of induced aromaticity [3] as a special type of stabilization of a conjugated cyclic system (initially non-aromatic or even non-existent in a free state) through interaction it with a suitable mono-or polyatomic fragment resulting in the completion of closed electronic shell of the composed system. A well-known example of this phenomenon (referred to as metalloaromaticity [4]) is a stable -complex ( 4 C 4 H 4 )Fe(CO) 3 in which stabilization of the antiaromatic cyclobutadiene ring occurs via complexation with a Fe(CO) 3 group giving rise to planarization of the four-membered ring and equalization of its C-C bonds [4][5][6]. In a similar way, donation of electrons from the metal atoms into -systems of the basal cycles in complexes 1 -3 provides for the formation of three-dimensional aromatic structures attended by flattening of the cyclobutadiene [7] and cyclooctatetraene [8] rings and equalization of the CC bonds.…”

“…In accordance with the principle of isolobal analogy [9] an apical irontricabonyl group in 4 can be replaced without loss of aromatic stability in the formed isostructural compound by other isolobal groups with two skeletal electrons to form, for example, stable pyramidal ( 4 C 4 H 4 )Be -complex 5 [10]. Analogously, stabilization of the three-dimensional *Address correspondence to this author at the Institute of Physical and Organic Chemistry at Southern Federal University, 344090 Rostov-on-Don, Russian Federation, Russian; Tel: +7(863)2434700; Fax: +7 8632 434 667; E-mail: minkin@ipoc.sfedu.ru aromatic cyclopentadiene-based pyramidal complex 6 is achieved via capping the five-membered ring with a tricarbonylmanganese group [6,11] with one skeletal electron and in the complex 7 with the univalent alkali-metal atoms [10,12,13]. At the same time, the replacement of the basal C n H n rings in sandwich and pyramidal systems with isolobal B n H 2n borane fragments results in metalloboranes [6,14,15], which are isostructural and isoelectronic to the hydrocarbon analogues.…”

“…Analogously, stabilization of the three-dimensional *Address correspondence to this author at the Institute of Physical and Organic Chemistry at Southern Federal University, 344090 Rostov-on-Don, Russian Federation, Russian; Tel: +7(863)2434700; Fax: +7 8632 434 667; E-mail: minkin@ipoc.sfedu.ru aromatic cyclopentadiene-based pyramidal complex 6 is achieved via capping the five-membered ring with a tricarbonylmanganese group [6,11] with one skeletal electron and in the complex 7 with the univalent alkali-metal atoms [10,12,13]. At the same time, the replacement of the basal C n H n rings in sandwich and pyramidal systems with isolobal B n H 2n borane fragments results in metalloboranes [6,14,15], which are isostructural and isoelectronic to the hydrocarbon analogues. Thus, metal-containing groups stabilize not only the aromatic forms of hydrocarbons C n H n , but also the aromatic nido-form of isoelectronic boranes B n H 2n , which does not exist in the free state [6,14].…”

“…At the same time, the replacement of the basal C n H n rings in sandwich and pyramidal systems with isolobal B n H 2n borane fragments results in metalloboranes [6,14,15], which are isostructural and isoelectronic to the hydrocarbon analogues. Thus, metal-containing groups stabilize not only the aromatic forms of hydrocarbons C n H n , but also the aromatic nido-form of isoelectronic boranes B n H 2n , which does not exist in the free state [6,14].…”