Looking at the Sennheiser's product description on the Amazon site, it looks like the audio input to the base is analog audio, not TOSlink, so I don't think you can use that to hook up the Sennheisers. You should be able to find a TOSlink to Analog adapter, but apparently your TV does not include a headphone jack.
Regarding your TOSlink to 3.5mm cable, the ones I've seen are intended to be used with the 3.5mm end on the source (Apple's AirPort, or my Pioneer portable DVD player) and not the receiver. There's no reason you couldn't do that, but I haven't seen that option before.
edit: I found this all-in-one cable to go from TOSlink optical to 3.5mm analog stereo on Amazon:
It uses an open USB port on your TV for power, or it can be connected to a USB phone charger.