Rotating object around Z axis with lerp/slerp

Im trying to rotate my player to face where I last clicked. I’ve acutally manged to do this, but now I want to see the player rotate at a set speed instead of the sprite just changing rotation instantly.
Ive tried several methods I’ve found online, but none of them work for me. Here’s what I have

void Update()
   if (Input.GetMouseButtonDown (0)) 

            Vector3 diff = Camera.main.ScreenToWorldPoint(Input.mousePosition) - transform.position;
            float rot_z = Mathf.Atan2(diff.y, diff.x) * Mathf.Rad2Deg;
            transform.rotation= Quaternion.Euler(0f, 0f, rot_z - 90);

            Instantiate(ProjectilePrefab, transform.position, transform.rotation);

The code above works fine, but it shows no movement. I have tried to do this but the position is wrong and the rotation is instant as well:

Vector3 diff = Camera.main.ScreenToWorldPoint(Input.mousePosition) - transform.position;
var newRotation = Quaternion.LookRotation(diff);
newRotation.y = 0.0f;
newRotation.x = 0.0f;
transform.rotation = Quaternion.Slerp(transform.rotation, newRotation, Time.deltaTime * 30);

Any ideas?

So, finally found the answer. I still dont understand but I made it work

 if (Input.GetMouseButtonDown (0)) {
                    target = Camera.main.ScreenToWorldPoint (Input.mousePosition);
                    rotateToTarget = true;
                    print ("NEW TARGET: " + target);

            if (rotateToTarget == true && target != null) {                             
                    print ("Rotating towards target");

                    targetRotation = Quaternion.LookRotation (transform.position - target.normalized, Vector3.forward);
                    targetRotation.x = 0.0f;//Set to zero because we only care about z axis
                    targetRotation.y = 0.0f;

                    player.transform.rotation = Quaternion.Slerp (transform.rotation, targetRotation, Time.deltaTime * rotationSpeed);  

                    if (Mathf.Abs (player.transform.rotation.eulerAngles.z - targetRotation.eulerAngles.z) < 1) {

                            rotateToTarget = false;
                            travelToTarget = true;
                            player.transform.rotation = targetRotation;
                            print ("ROTATION IS DONE!");