«A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of ...»
Treatment of fac-[Re(CO)3(CH3CN)3]+ with various tridentate amine ligands has produced several novel compounds, which most likely arise from reaction of the coordinated nitrile with ligand terminal amines. The reactivity of coordinated nitriles is an important, wellstudied topic, and the reader is referred to several excellent recent reviews and articles for background information.17-20 An important goal is to interpret how structure affects the NMR spectra of the facReI(CO)3L]n complexes. Previous work having this goal benefited from the study of the interaction of the chloride anion with the protons of the Re–NH groups of fac-[Re(CO)3L]n complexes.10 The topic of metal complexes as anion receptors is currently under intense study.21Thus, we utilize here the same approach to evaluate the interaction of Cl– with some of the new fac-[Re(CO)3L]n complexes because these complexes have unusual NH groups. When discussing specific compounds, we generally do not use the fac- designation because all the new compounds have this geometry.
3.2 Experimental Section Materials. Re(CO)10, diethylenetriamine (dien), N,N′,N′′-trimethyldiethylenetriamine
tetraethylammonium chloride, AgPF6 and AgBF4 from Aldrich, N,N-dimethyldiethylenetriamine (N,N-Me2dien) from Ames Laboratories, N,N′,N′′-triethyldiethylenetriamine (N,N′,N′′-Et3dien) from City Chemical LLC, and N′-methyldiethylenetriamine (N′-Medien) from TCI America were used as received. Re(CO)5Br, and [Re(CO)3(CH3CN)3]+ salts were synthesized by using slight
spectrometer. Peak positions are relative to TMS or solvent residual peak, with TMS as reference. All NMR data were processed with TopSpin and Mestre-C software.
X-ray Data Collection and Structure Determination. Colorless single crystals were placed in a cooled nitrogen gas stream at 90 K on a Nonius Kappa CCD diffractometer fitted with an Oxford Cryostream cooler with graphite-monochromated Mo Kα (0.71073 Å) radiation.
Data reduction included absorption corrections by the multi-scan method, with HKL SCALEPACK.27 All X-ray structures were determined by direct methods and difference Fourier techniques and were refined by full-matrix least squares by using SHELXL97.28 All nonhydrogen atoms were refined anisotropically. All H atoms were visible in difference maps, but were placed in idealized positions, except for some on N, which were refined when their positions were not unambiguously predictable. A torsional parameter was refined for each methyl group. The anion in [Re(CO)3(EAE)]BF4 also exhibits both orientational and substitutional disorder. The F atoms of the BF4– occupy two sets of sites, and a site of apparent ~5% occupancy by Br– lies near the B position.
acetimidamide). N,N-Me2dien (15 µL, 0.10 mmol) was added to a solution of [Re(CO)3(CH3CN)3]BF4 (48 mg, 0.10 mmol) in 6 mL of acetonitrile, and the reaction mixture was heated at reflux for 24 h. The volume was reduced to ∼2 mL, and diethyl ether was added until a fine precipitate was just visible. The reaction mixture was allowed to stand at room temperature; colorless crystals were observed in 2-3 days (15 mg, 28% yield). 1H NMR signals (ppm) in DMSO-d6: 7.99 (s, 1H, NH), 7.28 (s, 1H, NH), 7.10 ( s, 1H, NH), 3.19 (m, 4H, CH2), 3.03 (s, 3H, CH3), 2.97 (m, 2H, CH2), 2.63 (m, 2H, CH2), 2.44 (s, 3H, CH3), 2.31 (s, 3H, CH3).
The product was characterized by single-crystal X-ray diffraction.
amino)ethyl)acetimidamide). A 10% excess of N,N′,N′′-Me3dien (16 µL, 0.11 mmol) was added to a solution of [Re(CO)3(CH3CN)3]PF6 (54 mg, 0.10 mmol) in 10 mL of acetonitrile. The reaction mixture was heated at reflux for 24 h. A white powder was collected, which later gave X-ray quality crystals upon slow evaporation of a methanol solution (29 mg, 48% yield). 1H NMR signals (ppm) in DMSO-d6: 7.17 (s, 1H, NH), 4.50 (s, 1H, NH), 4.22 ( m, 1H, CH2), 3.20 (m, 1H, CH2), 3.10 (s, 3H, CH3), 3.03 (m, 2H, CH2), 2.98 (s, 3H, CH3), 2.91 (d, 3H, N1CH3), 2.80 (m, 2H, CH2), 2.60 (m, 2H, CH2), 2.14 (s, 3H, CH3). The above complex can be prepared more readily and reliably by the following procedure: N,N′,N′′-Me3dien (14 µL, 0.10 mmol) was added to a solution of [Re(CO)3(CH3CN)3]PF6 (54 mg, 0.10 mmol) in 8 mL of acetonitrile, and the reaction mixture was heated at reflux for 16 h, whereupon a very small amount of a fine precipitate was visible. The reaction mixture was transferred to a vial and allowed to evaporate to dryness at room temperature. The residue was dissolved in methanol, an excess of diethyl ether was added, and the mixture was left at room temperature. The white precipitate that had aggregated into clumps after 2 days was scraped from the walls of the vial, placed on a filter, and washed with diethyl ether. The NMR spectrum of this precipitate was identical to that of the crystals described above. The BF4– salt could also be obtained by using a similar procedure, which yielded a white precipitate having an NMR spectrum identical to that of the PF6– crystals;
however, the precipitate could not be crystallized.
[Re(CO)3(MAEH)F]PF6. Crystallization of the precipitate obtained during one of the syntheses of [Re(CO)3(MAE)]PF6 produced two types of crystals. One type, which was more abundant, was that just described for [Re(CO)3(MAE)]PF6, as confirmed by measurement of the unit cell dimensions. The less abundant type of crystals (needle-like), characterized by singlecrystal X-ray diffraction, contained [Re(CO)3(MAEH)F]PF6. 1H NMR signals (ppm) of the mixture in DMSO-d6 attributable to [Re(CO)3(MAEH)F]PF6: 6.93 (s, N4H), 6.19 (s, N1H), 3.03 (s, CH3), 2.99 (s, CH3), 2.56 (d, N1CH3), 2.23 (s, C12H3).
amino)ethyl)acetimidamide). A 10% excess of N,N′,N′′-Et3dien (20 µL, 0.11 mmol) was added to a solution of [Re(CO)3(CH3CN)3]BF4 (48 mg, 0.10 mmol) in 6 mL of acetonitrile, and the reaction mixture was heated at reflux for 16 h. The volume was reduced to ∼2 mL, and diethyl ether (10-15 mL) was added, whereupon a fine precipitate formed. After about 30 min the precipitate was collected on a filter and washed with diethyl ether (24 mg, 40% yield). (When the precipitate was too fine to filter, crystals could be obtained by letting the mixture stand covered and undisturbed for 1 day.) Crystals suitable for single-crystal X-ray diffraction grew upon slow
evaporation of a solution of the compound in methanol. 1H NMR signals (ppm) in DMSO-d6:
7.03 (s, 1H, NH), 4.40 (s, 1H, NH), 4.15 (m, 1H, CH2), 3.42 (m, 2H, CH2), 3.20 (m, 2H, CH2), 3.08 (m, 5H, CH2), 2.80 (m, 2H, CH2), 2.62 (m, 2H, CH2), 2.18 (s, 3H, CH3). 1.23 (t, 3H, CH3), 1.14 (overlapping triplets, 6H, CH3). The PF6– salt could also be obtained by using a similar procedure, which yielded a white precipitate having an NMR spectrum identical to that of [Re(CO)3(EAE)]BF4.
Cl– Titrations. Aliquots of a NEt4Cl stock solution containing 5 mM of the desired facRe(CO)3(L)]+ complex in DMSO-d6 or acetonitrile-d3 was added to 600 µL of a 5 mM solution of the complex, giving 1 to 140-150 mM Cl–. The solution was monitored by 1H NMR spectroscopy upon each addition.
Addition of Base to [Re(CO)3(MAE)]PF6 and [Re(CO)3(EAE)]BF4. A 5 mM solution of [Re(CO)3(MAE)]PF6 crystals in DMSO-d6 (600 µL) was treated with aqueous sodium hydroxide (0.1 M, 10 µL), and the solution was monitored by 1H NMR spectroscopy. Similar experiments were conducted with a dilute NaOH solution (0.033 M, 10 µL) and with [Re(CO)3(EAE)]BF4 (0.1 M, 10 µL).
3.3 Results and Discussion Synthetic Results. New [Re(CO)3L]n products prepared from [Re(CO)3(CH3CN)3]PF6/BF4 in acetonitrile are shown in Scheme 3.1. In water starting with [Re(CO)3(H2O)3]+, the ligands in Scheme 3.1 coordinate unchanged in a tridentate fashion to form normal [Re(CO)3L]n products.10 Because dien and N,N,N′,N′′,N′′-Me5dien gave the same normal [Re(CO)3L]n products in acetonitrile starting with [Re(CO)3(CH3CN)3]PF6/BF4 (details not given) or in water starting with [Re(CO)3(H2O)3]+,10 the formation of different products in acetonitrile than in water for N,NMe2dien, N,N′,N′′-Me3dien, and N,N′,N′′-Et3dien is attributable to the presence in these ligands of both steric bulk and NH groups. The reaction pathways in acetonitrile leading to the compounds in Scheme 3.1 are best discussed after we describe the structures of the new [Re(CO)3L]n complexes.
Products obtained in the syntheses using acetonitrile as a solvent. The compound numbers correspond to the structures in Figures 3.1 and 3.2. Adventitious HF from the decomposition of PF6– transformed some of 2a to 2b.
Structural Results. All complexes reported here exhibit a pseudo octahedral structure, with the three carbonyl ligands occupying one face and having typical Re–CO bond distances.10 The remaining three coordination sites are occupied by three nitrogen atoms of novel ligands in [Re(CO)3(DAE)]BF4 (Figure 3.1), [Re(CO)3(MAE)]PF6 (Figure 3.2a), and [Re(CO)3(EAE)]BF4 (Figure 3.2c). However, in [Re(CO)3(MAEH)F]PF6, two coordination sites are occupied by nitrogens and the third by fluoride (Figure 3.2b). Crystal data and details of the structural refinement for all these complexes are summarized in Table 3.1. The atom numbering systems in the ORTEP figures are used to describe the solid-state data. All complexes in Table 3.2 have a five-membered chelate ring with comparable N1–Re–N2 angles.
ORTEP plot of the cation in [Re(CO)3(DAE)]BF4. Thermal ellipsoids are drawn with 50% probability.
ORTEP plots of the cations in (a) [Re(CO)3(MAE)]PF6, (b) [Re(CO)3(MAEH)F]PF6, and (c) [Re(CO)3(EAE)]BF4. Thermal ellipsoids are drawn with 50% probability.
R = (∑||Fο| - |Fc||)/∑|Fο|; bwR2 = [∑[w(Fο2 - Fc2)2]/∑[w(Fο2)2]]1/2, in which w = 1/[σ2(Fο2) + (dP)2 + (eP)] and P = (Fο2 + 2Fc2)/3, d a = 0.0368, 0.0192, 0.0151, and 0.0305 and e = 5, 5.2495, 4.1664, and 1.826 for [Re(CO)3(DAE)]BF4, [Re(CO)3(MAE)]PF6, [Re(CO)3(MAEH)F]PF6, and [Re(CO)3(EAE)]BF4, respectively.
Selected Bond Distances (Å) and Angles (deg) for [Re(CO)3(DAE)]BF4 (1),a [Re(CO)3(MAE)]PF6 (2), [Re(CO)3(MAEH)F]PF6 (3), and [Re(CO)3(EAE)]BF4 (4) bond distances
The molecular structure of [Re(CO)3(DAE)]BF4 (Figure 3.1) reveals that the bound tridentate DAE ligand is an addition product between N,N-Me2dien and acetonitrile. None of the Re-bound N’s are derived from acetonitrile. One of the two five-membered chelate rings anchored by the central N (N2) is terminated by N3, the original N,N-Me2dien primary amine N.
The relatively short N3–C10 bond (1.293(6) Å; C10 is the original nitrile C) indicates doublebond character and an sp2 N. The other chelate ring is terminated by N1, the original N,NMe2dien tertiary amine N.
In [Re(CO)3(MAE)]PF6 (Figure 3.2a), one terminal amine N of the N,N′,N′′-Me3dien ligand has now become an endocyclic nitrogen (N3) having three bonds to C and no NH bonds.
This N has characteristics of an sp2 N and is part of a seven-membered chelate ring terminated by N4 (the original nitrile N) and anchored by N2, the original N,N′,N′′-Me3dien central N. N2 anchors the other chelate ring, and thus an uncommon five-membered/seven-membered chelate ring combination is created.
The bond lengths and angles centered on N3, N4 and C11 show that these are sp2 hybridized (relatively planar, bond angles near 120°) and that the N4–C11–N3 grouping exhibits electron delocalization. The N4–C11 (1.318(3) Å) and N3–C11 (1.344(3) Å) bonds show doublebond character. [Re(CO)3(EAE)]BF4 (Figure 3.2c) has a structure very similar to that of [Re(CO)3(MAE)]PF6, differing only in having ethyl groups instead of methyl groups at N1, N2, and N3.
Compared to [Re(CO)3(MAE)]PF6, the molecular structure of [Re(CO)3(MAEH)F]PF6 (Figure 3.2b) has the striking feature that N4 (derived from acetonitrile) is no longer part of an NH group bound to Re; the N4H has been protonated to become a dangling NH2 group, as found in [Re(CO)3(DAE)]BF4. This NH2 group forms a hydrogen bond to F, which is directly bound to Re.
Other complexes are known to have a seven-membered chelate ring similar to the type
[Pt(NH=CPhNButCH2CH2NHBut)Cl2]; Natile and co-workers30 described this pseudo square planar PtII compound as the final product of the addition of N,N′-But2ethylenediamine to coordinated benzonitrile. The obvious difference in geometry between six-coordinate ReI tricarbonyl complexes versus four-coordinate PtII complexes might suggest that useful comparisons are not possible because octahedral complexes are usually subject to greater interligand steric interactions. However, ReI tricarbonyl complexes are sterically undemanding because of the small size of the CO ligands and the relatively long bonds made by ReI. Bond distances involving ReI are longer than those involving PtII.11,25,33,34 N–M–N bite angles for chelate rings in related PtII and ReI compounds are more acute in ReI compounds.11,35 We believe this is true because the chelate rings normally adopt a preferred conformation or pucker; this conformation fixes the N-to-N non-bonded distance. However, the angles in the PtII and the ReI seven-membered chelate rings being compared here have similar values, 89.0(7) and 90.47(7)°, in contrast to the normal situation. Also in contrast to the normal situation, the non-bonded distance between these N atoms is much smaller in the Pt complex (2.79(2) Å) than in the Re complex (3.178(3) Å). The seven-membered chelate ring may be inherently strained and, unlike in other cases, the longer Re–N bond distances seem to allow the ring to reduce strain, explaining the unusually long N-to-N non-bonded distance, the large N–ReI–N bite angle, and the low chelate ring pucker in ReI vs. PtII (Figure B.1, Supporting Information). This strain aids in the transformation of the seven-membered chelate ring to a five-membered chelate ring as discussed next.