C−H Activation Induced by Water. Monocyclometalated to Dicyclometalated: C∧N∧C Tridentate Platinum Complexes

Metalation of 2,6-diphenylpyridine (1) by potassium tetrachloroplatinate in acetic acid
gives a monocyclometalated chloride-bridged dimer 4. This dimer is split with CO to give a
kinetic product 9t with the incoming CO trans to the orthometalated carbon. The kinetic
product of cleavage is shown to be 16 kJ mol-1 higher in energy than the thermodynamic
product 9c, which has the CO trans to the pyridine nitrogen. The isomerization of 9t to 9c
is shown not to take place via an associative mechanism and, with analogue 11, is effectively
suppressed when excess chloride is added, implying that it takes place via a chloride
dissociation. The monocyclometalated 9 undergoes a second cyclometalation to give the
C∧N∧C dicyclometalated complex 15 in high yield. This second cyclometalation is brought
about by the simple expedient of adding water to the monocyclometalated precursor. The
addition of water is rationalized on the basis of needing to ionize the HCl byproduct of the
reaction. Using a substituted pyridine (5) analogous chemistry is observed. Single-crystal
X-ray structures of one of the intermediates (6) and one of the final products (15) have been
solved. Density functional theory calculations are used to rationalize the isomerizations of
the monocyclometalated intermediates and the need to ionize HCl in the second cyclometalation.