ENFOQUE ACTORES

vibration in the free amine is found in the range 3500 - 3300 cm

—1 S 6

while in salts it is found at ca. 2700 cm . Other bands

at 1070 - 1000 cm ^ and at 790 cm ^ (broad) are assigned to possibly a Zn-H-Zn stretching mode and to a Zn-H wagging mode respectively.

The product is considered to be polymeric due to its apparent insolubility in inert solvents. There are therefore two structural possibilities.

L1I1 might seem more favourable since the zinc atoms have a coordination number of four while in LJJ the coordination number of zinc is five. On the other hand, LJ1 has a zinc hydridic hydrogen skeleton which is similar to that of the zinc di­ hydride which is believed to have a three-dimensional polymeric structure with bridging Zn-H-Zn units on the basis of infrared

N. /

_{JJ _}

spectroscopy . y][ might be expected to show two infrared absorptions, one terminal Zn-H vibration in the

-1

-1

region of 1600 cm to 1700 cm and a Zn-H-Zn mainly stretching vibration which may be expected around 1300 - 1400 cm \ Til would be expected to show an infrared spectrum similar to that of zinc dihydride. The actual infrared spectrum of the complex

-1 to as a Nujol mull showed a broad absorption from 1800 cm

1200 cm ^ as well as other absorptions which were assigned to the organic part of the complex. Thus the infrared spectrum supports structure LJIJ . . Structure LU , does allow for the possibility of structural . isomers thus

N N s /

In,

In'

N T

LIV , has a different structure from UL with the diethylamine groups, represented by N, being alternately' above and below the 'plane' of the zinc atoms. This isomerism would be expected to complicate the infrared spectrum of the complex and it

might lead to severe broadening of the zinc-hydrogen vibrations due to the presence of the different Zn-H-Zn units.

During attempts to crystallise out the complex a solution of the complex in benzene was obtained and this was analysed by ‘*‘H n.m.r. spectroscopy. The resonance which was observed at 4.18 ppm was assigned to the hydridic hydrogen. A similar

resonance at 4.20 ppm observed in the n.m.r. spectrum of

[TriMEDZnH]^ was also assigned to the hydridic hydrogen. The

quartet at 3.78 ppm was assigned to the N-methylene protons. The resonance due to these same protons in the free amine occurs at 2.58 ppm. A resonance at 1.08 ppm was assigned to the methyl protons in the ethyl groups and in the free amine these are found to resonate at 1.04 ppm. A resonance at 2.82 ppm was assigned to the amino proton whereas in the free amine this resonance is to be found at 0.90 ppm. Thus the resonances due to the protons in the methylene groups and in the amino proton, of the product in solution are shifted by between

1 and 2 ppm downfield from those in the free amine while the methyl proton resonances have essentially the same shift in the complex and in the free base. The size of ths shift of the methylene and the amino protons shows that a strong co­ ordination bond is formed. Usually the observed difference in the chemical shift of protons in -CH X-*(X = N, 0) on complex- ation is of the order of 0.5 ppm. e.g. a difference of 0.1 ppm in the chemical shift of the methylene group protons in

-CH NMe. on complexation during the formation of [Me NCH CH N- (Me)ZnH)2* No l°w field resonances (5-8 ppm) were seen apart from that due to the benzene solvent.

This is the region in which resonances due to bridging hydridic hydrogen atoms might be expected to be observed thus ther-'e does not appear to be any Zn-H-Zn species in

v

solution. Most probably the hydridic species in solution is monomeric

and on removal of the solvent, the monomeric units join by-

forming Zn-H-Zn bridging, thus achieving coordinative saturation for zinc and leading to the formation of an insoluble complex.

3.2.2 Diethylamine and TMEDA

Since the diethylamine zinc dihydride complex was poly­ meric and insoluble, it was thought that the presence of TMEDA in the reaction mixture might give a less associated, more soluble product. The reaction of zinc dihydride with diethyl­ amine in the presence of TMEDA was carried out using benzene as a solvent so that there would not be any unwanted solvent effects and an approximate two-fold excess of TMEDA was used. The reaction was slow and a product was obtained in 30% yield as Et2NHZnH2 (TMEDAZnl^^ after 60 hrs. Chemical analysis of the product satisfied a stoichiometry of Et2NHZnH2 (TMEDAZnl^ )^

The n.m.r. spectrum of the product showed no spin-spin

coupling in the methylene protons in the TMEDA ligand which might have been expected if the TMEDA was acting as a mono- dentate 'dangling 1 ligand. TMEDA must therefore be acting as a bridging or chelating ligand. It is known that (en)Zn£l

87