How can I see an output from SCL?
The minimum frequency your I2C clock is capable of running at (see end for details) is far higher than you will be able to see - your eyes physically cannot see it blinking at >30[kHz]. At most you'll see the LED being slightly dimmer than expected as data is being sent - might look like a single blink for the entire I2C packet. 

Additionally if you have wired up your LED from the pin to GND, you will also see nothing as I2C is open-drain. That means it can only sink current. You would have to connect the LED with anode to +5V (via resistor), and cathode to the SCL pin. But again you wouldn't be able to see anything. I see it is not from your picture.

Furthermore, the I2C clock is not constantly running. The clock output is produced only when you send data. As your code never sends an I2C packet (the loop() function is empty), the I2C core will never generate a clock. 

Once you start generating packets, you still wont see the clock blinking at its actual frequency. You might see it "blink", but the duration of the blink will actually contain multiple I2C packets. This is exactly the same as how you can make an LED dimmer by using PWM - you see the dip in brightness, but you cannot see the actual PWM frequency.

If you want to view the clock signal, you will require some form of logic analyser.





Due to hardware limitations, the ATMega328 cannot run the I2C clock rate at 1Hz (using hardware I2C). Regardless of what the API you are using allows you to enter.

The formula for calculating baud rate is:

$$f_{SCL} [Hz] = \frac{f_{CPU}}{16 + 2\cdot TWBR\cdot Prescaler}$$

Where the \$TWBR\$ register is an 8-bit value. \$Prescaler\$ is at most 64, but in the case of Arduino is set to 1 (can be gathered from the source code calculation being used). This gives:

$$f_{SCL(min)} [Hz] = \frac{16[MHz]}{16 + 2\cdot 255\cdot 1} = 30.418 [kHz]$$

The setClock API is shockingly written as it provides no form of error checking. In your case when you try to set a value of 1, this will try to write a value of 7999992 to the 8-bit \$TWBR\$ register, causing an overflow that will actually set the I2C rate to 30.418kHz.