According to Wikipedia…
On 16 November 2009, Mercedes bought Brawn GP and announced that McLaren would buy back Daimler’s 40 percent share of McLaren in a deal which reported to be worth about £0.5bn. Mercedes presently owns around 11% of the group, as 29% has been sold back to the group.
The new engines come into place in 2013.
And McLaren didn’t put their own engine in the F1. That was BMW.