Angle syntax is not working correctly

I have a html page that contains the following code:

<button 
  class="button button-assertive button-full button-outline centro icon ion-plus"
  option-getter="getOpt(option, complemento)" 
  modal-select="" 
  modal-title="{{complemento.nome}}"
  ng-model="produto.adicionais[complemento.nome]" 
  options="complemento.complementos"
  option-property="nome" 
  multiple="{{ isMultiple(complemento) }}"
  has-search="true">

But in this part I call a function that returns true or false :

multiple="{{ isMultiple(complemento) }}"

It is not working, the function is not even called, the final HTML end up with the same code as a string.

I've tried using no quotation marks as follows:

multiple={{ isMultiple(complemento) }}

Returning "true" or "false" of the function happening the same thing.