Synthesis and Properties of Novel (Tricarbonyl)(heptalene)chromium Complexes

Abstract

A number of novel (tricarbonyl)chromium complexes of heptalenes 10 ± 13, 16 ± 20 and 23 ± 25 have been prepared by reaction of the heptalenes with [Cr(CO) 3 L 3 ] (LNH 3 , Py; cf. Schemes 3 ± 6). Surprisingly, the offstate complexes 17 and 19, in which the Cr(CO) 3 group complexes on the diester ring, have been obtained with excellent regioselectivity. The directing effect of ester CO groups on the regioselectivity of the Cr(CO) 3 coordination to heptalene rings has been discussed. These complexes undergo thermal rearrangements via 1,2-intra-ring shift and inter-ring migration of the Cr(CO) 3 fragment to give the thermodynamically more stable on-state complexes 16 and 27, respectively (cf. Schemes 8 and 9). The analogous thermal behavior of other prepared complexes has also been investigated. A new procedure for the selective preparation of complexes 10 and 13, in which the Cr(CO) 3 group is coordinated to the phenyl ring of the styryl substituent has also been developed (Scheme 7). The attachment of the Cr(CO) 3 fragment to the phenyl group has a visible influence on the UV/VIS behavior of the on-state complexes 10 and 13a, as well as on the photochemical behavior of the DBS isomers 13a/13b (cf. Scheme 10). Introduction. ± So far, quite little is known about metallocarbonyl complexes of heptalenes [1], mainly because of the difficulties associated with the synthesis of various substituted heptalenes and their relative instability (cf. [2] [3]). Nevertheless, a number of tricarbonylchromium complexes of heptalenes have been synthesized, and their dynamic behavior has been investigated (Scheme 1) [4].The following events have been established: 1) Two thermally inducible rearrangements proceed in mononuclear complexes 2a ± 2d and 5a ± 5d (Scheme 2): a) intra-ring 1,2-shift (IRS) of the Cr(CO) 3 group and b) inter-ring migration (IRM) of the Cr(CO) 3 group. Both migrations are accompanied by a cyclic CC-bond shift (DBS) at the heptalene skeleton. Direct transformations 2b to 2d, as well as 2a to 2c, without DBS has been excluded [4b]. The thermal inter-ring migration of the Cr(CO) 3 group within complex 5c to yield 5b has been the first example of an inter-ring haptotropic rearrangement in nonplanar complexes of bicyclic p-systems [4a].2) 1,2-Haptotropic shifts and the inter-ring haptotropic migrations of the Cr(CO) 3 group are intramolecular processes. Activation barriers for both processes are quite similar [4].3) The occupancy of all four peri-positions in complexes 5a ± 5d by Me groups leads to activation barriers close to those for purely thermal decomplexation reactions [4a].4) The thermodynamically most stable complex 2a carries the coordinating Cr(CO) 3 group at the seven-membered ring, which is substituted with three electrondonating Me groups [4b].

Tunable haptotropic metal migration in fused arenes: towards organometallic switches

“…The haptotropic isomerization of naphthalene derivatives involves a metal shift between two adjacent aromatic rings. A similar metallotropic shift is known for the heptalene system [28][29] and for complexes containing an eight-membered carbocycle. 30,31 However, the coordinated aromatic rings of the starting material and product need not necessarily be adjacent to each other.…”

Coordination and Haptotropic Migration of Cr(CO)3in Polycyclic Aromatic Hydrocarbons: The Effect of the Size and the Curvature of the Substrate

“…This kind of reaction offers a plethora of possibilities in organic synthesis. Haptotropic shifts play an important role in reactions with electronically saturated complexes − and associative substitution reactions. , Remarkably, Dötz and co-workers have designed stereospecific molecular switches based on a reversible thermo- and photoinduced haptotropic shifts of the Cr(CO) 3 fragment along a naphthohydroquinone skeleton. ,, Some other important contributions addressed the topic of the diastereoselectivity in this reaction mechanism. ,− Recently, the selective migration of the Cr(CO) 3 group between two heterocyclic and phenyl neighboring rings through a combination of thermal and acid/base changes on the course of the reaction was reported …”

“…Haptotropic shifts play an important role in reactions with electronically saturated complexes [8][9][10][11] and associative substitution reactions. 12,13 Remarkably, Do ¨tz and coworkers have designed stereospecific molecular switches based on a reversible thermo-and photoinduced haptotropic shifts of the Cr(CO) 3 fragment along a naphthohydroquinone skeleton. 2,14,15 Some other important contributions addressed the topic of the diastereoselectivity in this reaction mechanism.…”

Intramolecular Haptotropic Rearrangements of the Tricarbonylchromium Complex in Small Polycyclic Aromatic Hydrocarbons

“…Inter-ring haptotropic rearrangements (IRHRs) of transition-metal (TM) complexes in which the TM π-coordinated to the polycyclic aromatic hydrocarbon (PAH) migrates between different rings of the PAH have been intensively studied, both experimentally and theoretically, over the past three decades. − Although sigmatropic shifts (involving intramolecular formation/rupture of σ-bonds) were first studied, these thermally induced rearrangements became very popular for their applications in TM-mediated organic synthesis. − One of the reasons is that the interaction between the low-lying unoccupied orbitals of the TM moiety and the high-lying occupied π-orbitals of the PAH ,− results in an important charge transfer from the organic π-ligands to the TM that changes the reactivity of the PAH dramatically. − Another reason is the possibility to use IRHRs in the design of molecular switches. − …”