Preferred exocyclic-amino coordination of W(CO)5 on 2-aminopyrimidine and 4-aminopyrimidine vs. heterocyclic N-1 coordination for 2-aminopyridine

2-Aminopyrimidine (2-ampym) and 4-aminopyrimidine (4-ampym) coordinate to W(CO)5 predominantly via the exocyclic amino group (>91% in 10 min photolysis) rather than to the endocyclic N-1 position as found for 2-aminopyridine (2-ampy). Photolysis of W(CO)6 in acetone in the presence of these ligands forms amino-bound [W(CO)5(2-ampym)] and [W(CO)5(4-ampym)] complexes. Secondary photolysis generates 18% (1.0 h photolysis) [W(CO)4(2-ampym)] or [W(CO)4(4-ampym)], chelated via the exocyclic amine and the adjacent endocyclic position (N-1 and N-3, respectively). Only ca. 10% of the more unhindered N-1-bound W(CO)5(4-ampym) was detected compared to virtually complete coordination via the exocyclic amino group for [W(CO)5(2-ampym)]. Mmff94 calculations show that the W(CO)5 coordination to the exocyclic donor is favored by 98.8 and 95.6 kcal/mol over the adjacent endocyclic position in the 2-ampym and 4-ampym complexes, respectively. Calculated W–N bond lengths by the Mmff94 methods gave exo-amine W–N bond distances of 2.24 and 2.26 Å and theoretical adjacent endocyclic W–N bond distances of 2.37 and 2.35 Å (isomers not observed from photolysis) for the 2-ampym and 4-ampym complexes, respectively. A W–(N-1) bond of 2.28 Å for this isomer of [W(CO)5(4-ampym)] was calculated. All W–N bonds are near the 2.18–2.33 Å range (mean of 2.27±0.06) for [W(CO)5L] (L=pyridine, piperidine, glycine, 1-(2-py)-1,2,4-triazole, [W(CO)5CN]−, 5-MeU−).