Head-to-Head (HH) and Head-to-Tail (HT) Conformers of cis-Bis Guanine Ligands Bound to the [Re(CO)3]+ Core

• Published in: Inorganic chemistry 2004-03, Vol.43 (6), p.2087-2096
• Main Authors: Zobi, Fabio, Blacque, Olivier, Schmalle, Helmut W, Spingler, Bernhard, Alberto, Roger
• Format: Article
• Language: English
• Subjects: Crystallography, X-Ray; Guanine - analogs & derivatives; Guanine - chemistry; Hydrogen Bonding; Ligands; Models, Molecular; Molecular Conformation; Molecular Structure; Organometallic Compounds - chemistry; Rhenium - chemistry
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic035012a

We have prepared four complexes of the type [Re(guanine)2(X)(CO)3] (guanine = 9-methylguanine or 7-methylguanine, X = H2O or Br) in order to understand the factors determining the orientation of coordinated purine ligands around the [Re(CO)3]+ core. The 9-methylguanine ligand (9-MeG) was chosen as the simplest N(9) derivatized guanine, and 7-methylguanine (7-MeG) was chosen because metal binding to N(9) does not impose steric hindrance. Two types of structures have been elucidated by X-ray crystallography, an HH (head-to-head) and HT (head-to-tail) conformer for each of the guanines. All complexes crystallize in monoclinic space groups:  [Re(9-MeG)2(H2O)(CO)3]ClO4 (2) in P21/n with a = 12.3307(10) Å, b = 16.2620(14) Å, c = 13.7171(11) Å, and β = 105.525(9)°, V = 2650.2(4) Å3, with the two bases in HT orientation and its conformer [Re(9-MeG)2(H2O)(CO)3]Br (3) in P21/n with a = 15.626(13) Å, b = 9.5269(5) Å, c = 15.4078(13) Å, and β = 76.951(1)°, V = 2234.5(3) Å3, and the two bases in an HH orientation. Similarly, [Re(7-MeG)2(H2O)(CO)3]ClO4 (4) crystallizes in P21/c with a = 13.0708(9) Å, b = 15.4082(7) Å, c = 14.316(9) Å, and β = 117.236(7)°, V = 2563.5(3) Å3, and exhibits an HT orientation and [ReBr(7-MeG)2(CO)3] (5) in P2/c with a = 17.5117(9) Å, b = 9.8842(7) Å, c = 15.3539(1) Å, and β = 100.824(7)°, V = 2610.3(3) Å3, and shows an HH orientation. When crystals of any of these complex pairs are dissolved in D2O, the 1H NMR spectrum shows a single peak for the H(8) resonance of the respective coordinated purine indicating a rapid equilibrium between HH and HT conformations in solution. DFT calculations simulating the rotation of one ligand around its Re−N bond showed energetic barriers of less than 8.7 kcal/mol. We find no hypochromic effect in the Raman spectrum of 3, which showed base stacking in the solid state. Neither steric interactions nor hydrogen bonding are important in determining the orientation of the ligands in the coordination sphere.