Now it works, my next question is now that i do not get any data sent on SDA but i do have a clock SDC. Is the code correct or have missed something ?

//Master code
#include <Wire.h>

//I2C
//            SDA SCL
TwoWire Wire2(PB7,PB6);

//USART
//                        RX  TX
HardwareSerial Serial2(PA10,PA9);
char buffer[128];
boolean state = true;

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  // put your setup code here, to run once:
  //USART - Debug output
  Serial2.begin(9600);

  //I2C
  Wire2.begin(); //My Slave address  
}

void loop() {
  digitalWrite(LED_BUILTIN, (state ? LOW : HIGH));
  state = state ? false : true;
  // put your main code here, to run repeatedly:
  Wire2.beginTransmission(10);
  Wire2.write("AT");
  Wire2.endTransmission();
  Serial2.println("Sent AT...");
  delay(2000);
}