Can I power Ardiuno using buck converter IC 3485 or any other buck circuit?
If you want to go with LM3485 or any other switching buck controller, you will need a bunch of external components, but this will give you really good power efficiencies (probably more than 90%, if implemented correctly). However if you want a simple solution, I mean if you don't want to mess up with a lot of external components (e.g. inductors, capacitors, diodes, FETS) you can just use a linear regulator with 6 V output regulation. 

As a choice for linear regulator, I can suggest using MC7806 (http://www.onsemi.com/pub_link/Collateral/MC7800-D.PDF), which will give you 6 V constant output voltage easily, or I can suggest using LT1086 (http://www.linear.com/product/LT1086) which can give you 5 V constant output easily or an adjustable output which you can set to 6 V. However notice that, when you are using a linear regulator your efficiency will not be very good. For example, if you want to make a 6 V output from 12 V input, your efficiency will be approximately Vout/Vin = 6 V/12 V =  0.5 = 50%, which means half of the power your battery supplies will be dissipated as heat, e.g. will be wasted. 

Also, besides these, you can just use the 12 V battery output to directly power your Arduino board through its power jack, which means you would not need to reduce 12 V into 6 V, if it was only for powering your board.