WIKI I2C
The maximum number of nodes is limited by the address space, and also by the total bus capacitance of 400 pF, which restricts practical communication distances to a few meters.
CAT5 CAPACITANCE
52pF / metre

cat5 balanced cable / i2c unbalanced signal.
cat5 characteristic impedence of 100 ohms is probably a big load for the i2c pins also.
its not a good match , and won't go far