there is no such thing as a PCI-e 2.1 slot, only PCI-e 2.0 slots.
read more in the website i have linked:
PCI Express 2.1PCI Express 2.1 supports a large proportion of the management, support, and troubleshooting systems planned for full implementation in PCI Express 3.0. However, the speed is the same as PCI Express 2.0. Unfortunately, the increase in power from the slot breaks backwards-compatibility between PCI Express 2.1 cards and some older motherboards. Most motherboards sold currently come with PCI Express 2.0 connectors.