.titleicon {
  background: red;
  position: absolute;
  width: 100%;
  left: 0;
  bottom: 0;
  text-align:center;
}

<ion-header>
  <ion-toolbar>
    <ion-buttons slot="start" style=" position: absolute;">
      <ion-menu-button></ion-menu-button>
    </ion-buttons>

    <div class="titleicon">
      <img src="../assets/img/logotipo-areco.jpg" width="100px" />
    </div>
  </ion-toolbar>
</ion-header>