While strong catenation is almost exclusively reserved for C, some transition and post-transition elements can oftentimes take part in delocalized electron systems and ring systems. But does the two come together, that is, is there any known aromatic compound that has two adjacent transitional/post-transitional elements in aromatic ring as heteroatoms? If Yes or No, then Why?
Suppose for a 5-valency element, there are two mutually perpendicular pi-orbitals (along with a sigma molecular orbital) that make a triple bond between two atoms, remaining two valency "hands" are used up for taking part in two mutually perpendicular aromatic ring system. Keeping in mind instability of benzyne and ONC (Octanitrocubane), would that structure be chemically stable and permissible by quantum chemistry? If yes, then what kind of novel properties can be expected from the compound and what would be the stability of it? What would the most suitable element for this purpose?
Attached with it is a Lewis structure of a tentative complex of X metal. Note, the two aromatic structures are not coplanar, rather they are orthogonal
Could you please draw what you are having in mind?
I got pretty lost in the description. My guess is that you mean smth of a kind of metallabenzene dimers with biphenyl structure, with metal=metal bond in the middle
https://en.wikipedia.org/wiki/Metallabenzene
Actually, I can recommend review Article Metallaaromatic Chemistry: History and Development
Despite it does not contain such structures.
Looking from the side of electron configuration: if TM-atom (Transition metal) becomes part of aromatic ring it suppose to provide not 2, but 3 electrons - 2 for making sigmabonds and one for donation in pi-system. Actually we can imagine situation, when not 1, but 2 or 0 electrons are provided to conjugation (in 5 or 7-membered rings), but still such atomic orbital, perpendicular to plane of aromatic ring should exist. (We can speculate, based on the angle between sigma bonds, that there is a place just for one more orbital, that would lie in the plane of the aromatic ring and directing out from it, but I would skip this argument). Most important is that such orbital is perpendicular to metal-metal bond. At the same time, triple metal-metal bond requires 2 pi-bonds, building of each requires from each atom 2 orbitals lying perpendicular to the sigma bond metal-metal. Thus 2 options can be imagined and both are bad:
1) there are 3 dumb-bell-like orbitals on each atom in one plane, that is perpendicular to metal-metal bond, while 3 other assymetric dumbbells make sigma-bonds. Thus we are speaking about the angle between 3 orbitals in plane around 60 degrees and situation comparable to a quaternary bond. VSEPR theory does not know such creatures.
2) each of pi-bonds of metal-metal triple bond is conjugated with aromatic system, so kind of "hetero-fulvene". In this case plane of one aromatic ring would perpendicular to one another ring, but pi-bond TM-TM conjugated with second ring would lie in this plane. Assuming that orbital forming pi-bond is symmetric dumbell, and angle between sigma-bonds of aromatic ring is >120 degree (while TM is bigger than C), angle between pi-orbital and each of this sigma would be below 30 degrees. That is again impossible.
Dear Sumit Bhowmick this is certainly a very interesting and unusual technical question. I have no definite answer to this question and can only speculate about a possible solution. Formally such a molecule would be a "dimetallanaphthalene". I searched the SciFinder for the term "dimetallanaphthalene" and received exactly 0 (zero) answers. Apparently such a molecule has never been prepared experimentally. In my opinion the most suitable element X for forming such a structure would be a Group 6 metal (chromium, molybdenum, or tungsten). These metals are well-known for forming stable "lantern-like" complexes with bridging ligands and multiple bonds between the metal atoms. even quintuple bonds between two Cr or Mo atoms are possible:
https://en.wikipedia.org/wiki/Quintuple_bond
(see attached diagram)
This question intrigued me.
The compound you have in mind will not be aromatic. I tried it with phosphorous atoms. In this case you clearly have 2 lonepairs and no triplebond. Heavier atoms will probably be isolobal. Only if you block the p orbitals with additional substituents/ligands you can force electrons to use higher orbitals. Anyway your multiple bond is then constructed with delta-orbitals or even more exotic.
Another aspect is, that you have to consider canonical MOs. They will not be restricted to one of the 2 rings (its a question of the pricipal axis)
I have attached you a file containing a NICS analysis where I compared your compound with the vogel aromat (methano[10]annulene). Also the HOMOs are included. You can clearly see, that your compound, if existing, would be antiaromatic and the HOMO is completly delocalised.
For the calculations I used B3LYP/6-31G(d,p) with g09
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