Question on VO(acac)-piperidine

[warenzeichen]

I am curious of the reason why the V=O stretching here has a lower frequency than that in VO(acac)2. Is it related to resonance? I am not sure.

 

[alxm]

Yeah, you can explain it in terms of resonance.

Basically (ignoring the two existing bonds to vanadium) you have V=O <--> -V-O^- where in the latter case, some ligand forms an additional bond to the vanadium atom, creating a cation somewhere else.

In the case of VO(acac)₂, an oxygen forms an additional bond, leaving a cation on the carbon it is bonded to. With piperedine, the nitrogen can form a second bond to vanadium, with its lone pair, making the nitrogen a cation.

The nitrogen cation is more stable than the carbocation, so the V-O^- resonance form is more stable with piperidine, so the V=O double-bond is weaker, so the V=O stretch has a lower frequency.

(Updated: I did a calculation, it's a pretty small shift, about 20 cm^-1? http://i38.tinypic.com/25z5pwg.gif")

 

[warenzeichen]

about 40 cm^-1

the V=O stretching vibration for VO(acac)2 at 995cm^-1 and for VO(acac)-piperidine it is at 957cm^-1

I think there should be four bonds except the V=O ? also, one acac is substituted by piperidine, isn't it?

Thanks.

 

[alxm]

about 40 cm^-1

Well, close enough - well within the margin of error. :)

I think there should be four bonds except the V=O ? also, one acac is substituted by piperidine, isn't it?

Well, with VO(acac)₂ you'll have four oxygens liganding, but with two bonds which you could view as resonating between the two acetylacetonate oxygen atoms. So, 'half' a bond to each oxygen would really be a better way of thinking of it. (and in the zwitterion-resonance form, you'd have two full bonds to one acac, and a carbocation on the same acac) With VO(acac) you'd replace one acetylacetonate with the piperidine (CH₂)₅N^-.

 

[warenzeichen]

Thanks for your answer.
and finally could you tell me how to calculate it ? It seems that I can not calculate it based on the information I knew in the IR( I don't learn the IR on metal in details... But it looks pretty similar to IR but in more complex way...)

 

[ChemHater]

about 40 cm^-1

the V=O stretching vibration for VO(acac)2 at 995cm^-1 and for VO(acac)-piperidine it is at 957cm^-1

I think there should be four bonds except the V=O ? also, one acac is substituted by piperidine, isn't it?

Thanks.

Hello there,

The stretching frequency of v(V=O) band is 957 cm^(-1), could you please tell me if it is your experimental value or literature value?

Thanks