public Envelope Extent {get; set;}

var layout = LayoutView.Active.Layout;
var mf = layout.GetElementsAsFlattenedList().OfType<MapFrame>()
  .First(mf => mf.Name == mapFrame);
var autoCamera = mf.GetAutoCamera();
autoCamera.Source = AutoCameraSource.Fixed;
autoCamera.AutoCameraType = AutoCameraType.Extent;

var mf_extent = mf.GetViewExtent();

var extent = EnvelopeBuilderEx.CreateEnvelope(
  400748.62, 800296.4, 1310669.05, 1424520.74, mf.Map.SpatialReference);
autoCamera.Extent = extent;

if (mf.ValidateAutoCamera(autoCamera) &&
  !mf.IsMapSeriesMapFrame())
  mf.SetAutoCamera(autoCamera);