How can I set center anchors for ScrollView property "contentitem"?

ningen

I want set rectangle position to application window center
But there is always only top left anchors. My code is:
@import QtQuick 2.2
import QtQuick.Controls 1.1
import QtQuick.Window 2.0
import "qrc:/qml"
ApplicationWindow {
visible:true
id:wp
ScrollView
{
id: wpScroll;
anchors.fill: parent
contentItem: Rectangle
{
width: 800
height: 800
color : "yellow"
anchors.centerIn: wp.center ///wp=:ApplicationWindow

    Rectangle{
        width: 200;
        height: 200;
        color:"red"
        anchors.bottom:  parent.bottom
    }
}

}@

!http://www.7image.ru/pics/1214/469892113.png(Yellow Rect is top left align instead of centered position)!