CHEM4680 - Organometallic Chemistry
Mid-term test: electron counting, main group metals and NMR
General remarks:
· You get marks for your arguments, not for the answers themselves. A correct answer without the relevant argument doesn't get you any marks. A "wrong" answer arrived at by reasonable arguments gets you at least some points, possibly even full marks.
· Questions 1-3 count for 40/40/20 of the total score. Within each question, sub-questions have identical weights.
· Please note name and student nr on each sheet you use for answers.
· Success !!!
1. Counting.
a. Give electron count and oxidation state for the following species. Try to draw conclusions regarding stability and possible rearrangements, based on count, oxidation state and any other relevant aspects you can think of.
· H2Se
· Me3NGaMe3
· (Me3CCH2)3Ta=CHCMe3
·
·
2. Main group metals
Compounds 1a-d (see below) react in different ways with excess BuLi, depending on the nature of X. Treatment of 1a first with an excess of BuLi and then excess of Ph2PCl gives nearly pure 2a, whereas the same treatment of 1b gives mainly 3b.
a) Explain how 2a and 3b are formed, and explain the observed selectivity.
b) What would you expect to obtain from 1c after similar treatment?
c) Reaction of 1d is not very selective. Which products would you expect on treatment of this compound first with BuLi, then with water? Propose at least two possibilities, formed from two different reactions with BuLi.
3. NMR
Complex 4 was recently studied by Frost and Mebi. The complex is soluble in water.
In THF-d8 containing a trace of water, its 1H NMR spectrum shows a characteristic hydride resonance at 14.4 ppm, as a triplet (JPH = 37 Hz). At elevated temperature, the 1H resonance broadens and then sharpens to a singlet.
If the compound is dissolved in D2O and heated, the hydride peak simply disappears, the intensity of the HOD peak (always visible in D2O) increases. The 31P NMR spectrum shows a doublet at 12 ppm (JPH = 37 Hz). After heating in D2O and cooling again, this is replaced by a 1:1:1 three-line pattern with a spacing of ca 6 Hz.
Note: 1H and 31P have nuclear spin 1/2, D (same as 2H) has nuclear spin 1.
a) Explain what must be happening to complex 4 on heating in wet THF and in D2O (overall reactions, not a mechanism yet).
b) The reaction is accelerated by acid. Can you propose a mechanism?
-- 3 --