I have used the I2C commands at 100KHz and 400KHz. I have never used it at 1MHz. I'm not sure if you can use the I2C commands at 1MHz. Below, there is a quote from the PBP help.

The timing of the I2C instructions is set so than standard speed devices (100KHz) will be accessible at clock speeds up to 8MHz. Fast mode devices (400KHz) may be used up to 20MHz.