3.2 Amide based pincer chemistry
3.2.2 Dinitrogen chemistry with [ReCl 2 (PNP iPr )] (30)
30 shows a high tendency to bind a sixth ligand, as exemplified by the formation of a THF complex or the non-trivial synthesis of the penta-coordinated complex 30. This can be
96 Chapter 3 Rhenium complexes ofiso-propyl based PNP pincer ligands for dinitrogen activation
attributed to the reduced steric shielding of the metal center and is in line with previous results on the corresponding [RuCl2(HPNPR)] complexes (R =iPr,tBu).209,210Accordingly, when 30 is exposed to an N2 atmosphere, the color changes immediately from deep violet to brown-orange, indicating fast reaction with dinitrogen at the Re(iii) oxidation state. The NMR spectra reveal largely broadened signals, with the 31P{1H} NMR signal vanishing almost in the spectral noise, at a similar shift as pure30. However, if the sample is cooled (−15 - −50 °C), a new set of signals forms (see Figure 3.12 a). Warming the sample back to RT results in reformation of the previous, broad signals and when the sample is heated (up to 75 °C) or when Ar is bubbled through the solution, 30 becomes the main species again. Hence, the reaction with N2 is dynamic, fully reversible and thus most likely mere coordination chemistry. The following discussion therefore focuses on the identity of the species which are present at low temperatures under an N2 atmosphere.
The 31P{1H} NMR spectrum reveals five new signals, i.e. two doublets at δ31P = 1.3
Fig. 3.12. Low temperature NMR spectroscopy of the reaction of 30 with N2. a) 31P{1H} NMR spectrum, showing three pincer fragments. Coupling constants are given in Hz. b)
15N{1H} NMR spectrum, recorded inverse gated. c)1H-31P heteronuclear multiple bond correlation (HMBC) revealing both, fragment B (turquoise) and C (red) to feature a mirror plane within and orthogonal to the pincer plane, respectively. For better readability, the1H NMR trace was substituted with the1H{31P} NMR spectrum.
3.2 Amide based pincer chemistry 97
and −1.2 ppm with a mutual coupling2JPP = 276.2 Hz (denoted A in the following), two doublets at δ31P = −10.8 and −14.1 ppm with a mutual coupling of 2JPP = 253.0 Hz (denoted B) and a singlet at δ31P = −16.4 ppm (denoted C), indicating three different PNP-pincer ligands, two with inequivalent phosphorous atoms as well as one with equivalent phosphorous atoms. Importantly, the ratio B:Cwas found to be close to 1:1 in every single spectrum that was measured, while the ratio A:B/C changed between the experiments.
The coupling constants are typical fortrans coupling of the pincer ligand in an asymmetric environment, whereas the constant 1:1 ratio of B and C indicates either some kind of connectivity between these fragments or a dissociation of one intermediate into two products.
The origin of these varying ratios was not investigated, yet it is noteworthy that Awas the minor product in all measurements and most of the times the signals were less intense than in the exemplary spectrum in Figure 3.12a.
Therefore the focus of further investigations was on the nature of B and C, which was further elucidated using 2D-NMR spectroscopy. By1H-31P HMBC spectroscopy, coupling of B andCto two inequivalentiso-propyl groups (one CH and two CH3 signal) each could be detected. Therefore, both pincer ligands are coordinated in aCSsymmetric environment, but with orthogonal mirror planes (i.e. once within the ligand plane (B) and once perpendicular to it (C). An identification of theiPr signals belonging to the31P signalAwas not possible, due to the low concentration. If 30 was placed under an atmosphere of 15N2 gas, at low temperatures three signals were identified by15N{1H} NMR spectroscopy atδ15N=−4.1 ppm (dt,J = 5.5 and 2.6 Hz), −12.7 ppm (s) and −18.8 ppm (d, J = 5.7 Hz with non-resolved shoulders which can be estimated to a coupling constant of ≈1.5 Hz) (see Figure 3.12 b).
Again, the signals atδ15N =−4.1 and−18.8 ppm integrate in a 1:1 ratio, while the signal at δ15N=−12.7 ppm integrates to 0.4. This matches the ratio betweenB/Cvs. Adetermined by31P{1H} NMR spectroscopy. The coupling constants indicates the resonances at δ15N =
−4.1 and−18.8 ppm to belong to a single, asymmetrically coordinated N2ligand, while the signal at−12.7 ppm must belong to an N2ligand in a symmetric coordination environment.
Notably, under 15N2, in the 31P{1H} NMR spectrum the resonance at δ31P = −16.4 ppm splits into a doublet with JPN = 2.6 Hz, matching the small coupling of the signal at δ15N
=−4.1 ppm and thereby proving binding of the N2 ligand to fragmentC.
Finally, the sample was subjected to an 1H{31P} NOESY as well as a 31P{1H} DOSY measurement (see Figure 3.13). The NOESY revealed small cross peaks betweeniPr groups belonging to Band C, indicating both fragments to actually belong to a single compound 33a. This was confirmed by the low temperature 31P-DOSY measurement, which was performed first at -30 °C on a sample of 30in toluene under Ar. Then N2gas was bubbled through the solution and the sample was remeasured under N2 at the same temperature.
The data shows all three pincer moieties to have the same diffusion coefficient of D = 1.89·10−6cm2s-1, while pure 30has a coefficient of D = 2.61·10−6cm2s-1. Since these values are derived in toluene-d8 at -30 °C, for comparision with the previously published diffusion constant oftert-butyl substituted complexXIX (D=9.1·10−6cm2s-1 in THF-d8
98 Chapter 3 Rhenium complexes ofiso-propyl based PNP pincer ligands for dinitrogen activation
-10 ö÷ ø
0 öø
10 ø
δ / ppm
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þÿ-1 ) 1.0 1.1
1.2 1.3
1.4 1.5
1.6 1.7
1.8
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δ / ppm
1.0
1.5 δ (ppm)
Fig. 3.13. Low temperature1H-NOESY (top) and31P-DOSY (bottom) of the reaction of30 with N2in toluene-d8. The NOESY shows cross peaks betweeniPr groups belonging to the fragmentsBandCwhile the DOSY shows smaller diffusion coefficients forA,BandC (blue), compared to30(red) (all measured at -30 °C).
at RT)[95] theStokes radii for all three diffusion constants were calculated according to the Stokes-Einsteinequation:
D= kBT
6πηr (3.1)
where D is the diffusion constant, kB is Boltzmann’s constant, T is the temperature, η is the dynamic viscosity of the solvent at that temperature and r is the Stokes radius, i.e.
the radius a spherical particle with that D would have. The restriction of this equation to spherical particals limits its use for quantitative radii determination, however since the molecules discussed here do not deviate too much from that geometry, eq. 3.1 can be used in reasonable approximation of qualitative analysis. With ηtoluene, -30 °C = 1.39 mP s and ηTHF, 25 °C = 0.456 mP s,211,212 the following Stokes radii could be calculated: r(XIX) = 4.95 Å, r(30) = 4.48 Å and r(33a) = 6.69 Å.
3.2 Amide based pincer chemistry 99
R
Fig. 3.14. Summary of structure determination of 33a and 33b by low temperature NMR spec-troscopy and computational analysis of the thermochemistry of the formation of33aand 33bat different temperatures.
All these data form a coherent picture of the reaction of30 with N2, which is summerized in Figure 3.14:
• The reversible nature of the reaction tendentially indicates simple coordination of N2. The formation of the product(s) only at low temperatures points to an entropic penalty of the reaction, in line with a dimerization process.
• 31P NMR as well as 1H-31P HMBC spectra allow for identification of three pincer fragments and their symmetry planes.
• 15N NMR spectroscopy proves N2 binding to fragment C, while binding of the same ligand to B is suggested by a triplet-like shoulders of the second signal of similar intensity. If Acoordinates N2 it has to be bound symmetrically.
• NOESY and 31P-DOSY reveal all three fragments to belong to dimers, withB andC actually being two sides of the same dimer.
Consequently,Amost likely corresponds to [(µ-N2){ReCl2,cis(PNPiPr)}2] (33b), a symmetric analog of the previously observed tert-butly substituted analog XX (see Section 2.1) but with additional chloride ligands trans to the N2 bridge. On the other hand, B and C are the two sides of an asymmetrical dimer [{ReCl2,cis(PNPiPr)}(µ-N2){ReCl2,trans(PNPiPr)}]
(33a) as depicted in Figure 3.14. These suggestions are supported by DFT calculations,5 which reveal the binding of N2 to form dimer 33a and 33b to be favorable at −30 °C by
∆G0 = −19.6 and −25.3 kJ mol-1, respectively. At 25 °C, the reaction is computed to be almost thermoneutral (∆G0= 2.2 and−2.7 kJ mol-1), fully consistent with the experimental observation. Coordination of N2to30eithercis ortrans to the amido ligand was calculated to be uphill in all cases. Unfortunately, neither 33anor 33b could be crystallized to allow for X-ray diffraction, despite various attempts. Hence, no molecular structures could be determined experimentally.
5PBE0/D3BJ/RIJCOSX/def2-TZVP/CPCM(toluene)||PBE/D3BJ/RI/def2-SVP
100 Chapter 3 Rhenium complexes ofiso-propyl based PNP pincer ligands for dinitrogen activation