Dynamic Pathways for Fluxional Molecules Defined Using Exchange-NOE Peaks

Abstract

In examining NMR methods to assess orientation and fluxional motions of planar N-donor heterocyclic coordinated ligands (L's) in solution, we introduce the use of exchange-NOE NMR data as a powerful method for defining (i) the extent of L rotation about the metal-N bond, (ii) the direction of L rotation, and (iii) even the halves of C 2 -symmetrical L's that interchange during dynamic processes. The full value of the approach depends on a strategy in which the complexes studied are chiral and similar except that one has a lopsided L (Me 3 Bzm ) 1,5,6trimethylbenzimidazole in [Re 2 O 3 Cl 4 (Me 3 Bzm) 4 ] ( 1)) and the other has a C 2 -symmetrical L (3,5-lut ) 3,5-lutidine in [Re 2 O 3 Cl 4 (3,5-lut) 4 ] (2)). Each Re is part of a nearly linear OdRe-O-RedO grouping and has a "terminal" L (L t ) and a stacked L (L s ). The fluxional inversion of the two chiral dimers involves rotations of ∼180°about the Re-O-Re bonds and of ∼90°about all four Re-L bonds. The exchange-NOE data for 2 show that the half of L t away from the dimer center interchanges with the half of L s close to the center, with the L plane rotating past the OdRe-O bonds, not the N-Re-Cl bonds. Thus, the exchange-NOE data help to establish the direction of L rotation. In 1, Me 3 Bzm t has the rare head-to-head (HH) orientation with respect to the partner. This partner Me 3 -Bzm s stacks with the Me 3 Bzm s from the other Re in the common head-to-tail (HT) orientation. Compelling evidence that the predominant solution conformer of 1 has the HH,HT,HH structure includes unusual chemical shift dispersions and a strong interligand NOE cross-peak. This is the only case in which cis,bis imidazole-ring-ligated untethered ligands have been found to be predominantly HH in solution. This predominance can be attributed to the electrostatic attraction of the δ + N 2 C proton for the negative core of the molecule (bridging O, cis Cl on same Re, and two cis Cl's on the other Re).

Slowly Exchanging cis‐PtA2B2 Atropisomers with A2 = Chiral Diamine Ligands and B = 5,6‐Dimethylbenzimidazole

“…[22,27] [22] one HT and one HH form were observed in solution. For the HH conformer, the H2 signal of the uncanted Me 3 Bzm base that is cis to the Cl atoms was 1.5 ppm downfield with respect to the H2 signal of the canted Me 3 Bzm base that is cis to the Me 2 SO groups.…”

Re(V) Complexes with an Open-Chain Quadridentate Ligand Containing Two Amine and Two Amido Donors. Synthesis, Characterization, and Solution Equilibria of Re2O3(dioxo-tetH4)2 and [ReO(H2O)(dioxo-tetH4)]Cl (dioxo-tetH6 = 1,4,8,11-tetraazaundecane-5,7-dione)

“…Stacking of the two nonterminal N-heterocyclic donor ligands (N) is a common feature in Re 2 O 3 Cl 4 (N) 4 type complexes. 30,33,34 Stacking was also observed in Re 2 O 3 (salpd) 2 (salpdH 2 is N,N′-propane-1,3-disalicylideneamine). In this case the terminal salicylideneamine chelate rings are planar and the central propyl chelate ring is puckered.…”

“…The Re-N bond distances are within the range expected for neutral amine, N(1) and N(4), and charged amido, N(2) and N(3), donor groups. 2, 30 The interior portions of the ligands are nearly planar (mean deviation of the leastsquares plane for C(2), N(2), C(3), C(4), C(5) N(3), C(6) is 0.0737 Å) and are stacked, but rotated by ∼45°with respect to each other so that the carbonyl groups and the flanking fivemembered chelate rings are staggered. Stacking of the two nonterminal N-heterocyclic donor ligands (N) is a common feature in Re 2 O 3 Cl 4 (N) 4 type complexes.…”

Ligands Rock & Roll: Stepwise Twisting of Two cis‐Coordinated Lopsided N‐Heterocycles in an Octahedral Bis(2‐phenylazopyridine)–Ruthenium(II) Complex with Seven Atropisomers

“…For complexes of the type cis-[Ru(bpy) 2 ], a-and b-[Ru(azpy) 2 ] being borderline complexes with respect to the stable cis bifunctional coordination to non-tethered purines, these studies prompted us to investigate this aspect using the model bases 1-methylimidazole, MeIm, and 1-methylbenzimidazole, MeBim. [31,32,44,48] 1-Methyl(benz)imidazole derivatives are useful DNA model bases in the investigation of the bifunctional binding of sterically encumbered octahedral complexes, [18,[24][25][26][27][28][29][30] and MeIm and MeBim form stable bisadducts with ruthenium bis(didentate) complexes. The imidazole H(ii) proton is structurally related to the H(8) atom of guanine derivatives, in addition, MeIm and MeBim possess a proton on the other side of the ligand, that is, H(iv), a most valuable (NMR) probe proton that is lacking in guanine derivatives (Scheme 1).…”

“…[19][20][21][22][23] 1 H NMR spectroscopy has been proven to be a powerful tool to study the rotational behavior of s-bound ligands in square-planar four-coordinate metal complexes. To date, however, only in a few suitable octahedral systems have the orientation and dynamic properties of s-bound N-heterocycles been studied, like dinuclear m-oxorhenium(v) complexes, [24][25][26] and dichlororuthenium(ii)-dmso complexes. [24,[27][28][29][30] More recently, we have introduced ruthenium(ii)-bis(bipyridine) complexes of the type cis-[Ru-(bpy) 2 (L) 2 ](PF 6 ) 2 of which the dynamic process of heterocyclic ligands (L) can be studied very well with different kinds of monodentate ligands, whilst the backbone consists of two static didentate ligands.…”