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 Angstrom and theoretical adjacent endocyclic W-N bond distances of 2.37 and 2.35 Angstrom (isomers not observed from photolysis) for the 2-ampym and 4-ampym complexes, respectively. Angstrom W-(N-1) bond of 2.28 Angstrom for this isomer of [W(CO)(5)(4-ampym)] was calculated. All W-N bonds are near the 2.18-233 Angstrom range (mean of 2.27 +/- 0.06) for [W(CO)(5)L] (L = pyridine, piperidine, glycine, 1-(2-py)-1,2,4-triazole, [W(CO)(5)CN](-), 5-MeU-). (C) 2001 Published by Elsevier Science B.V.