The electronic structure of 29

The strongly shifted NMR signals of29described in Section 3.1.1, in particular the presence of a ³¹P resonance at δ31P = −1525.9 ppm, are reminiscent of previously published work on the osmium(ii) PNP pincer complex [OsCl(P N P^tBu)].^[195] This complex exhibits a peak in ³¹P{¹H} NMR spectroscopy at δ31P = −978.2 ppm and a TIP, as indicated by a linear behaviour in theX_MT vs.T plot derived from SQUID magnetometry, with a constant susceptibility of XM(TIP) = 1030·10^-6cm³mol^-1. Such behavior has been ascribed to the second-order Zeeman effect which arises from mixing of the thermally isolated ground state (i.e. kBT <<∆E, with ∆E describing the energy difference between the ground state and

3.1 The starting platform - [ReCl₃(HPNP^iPr)] (29) 87

-d

29 29

Fig. 3.4. Left: Temperature-dependent molar susceptibility data of 29 determined experimentally by SQUID magnetometry (red) as well as computationally by QDPT/NEVPT2/CASSCF calculations (black). Right: ATR-IR spectrum of29(black), compared with the spectrum of29-d (blue).

the first excited state) with excited states in the presence of a magnetic field.^[203] Indeed, multireference calculations on the Os(ii) complex indicated the complex to exhibit rather strong SOC effects. A similar situation for 29 seems very likely, as TIP is a known phe-nomenon especially for octahedrally coordinated heavy metal complexes with a d⁴ electron configuration like Re(iii).^[204]

In order to gain insight into the electronic structure of the system at hand, its magnetic be-havior was evaluted by means of SQUID magnetometry (see Figure 3.4). Over a temperature range of 2-295 K, complex29exhibits a strong TIP as indicated by a linearly increasingX_MT vs. T curve, corresponding to a molar susceptibility of X_M(TIP) = 1063.4·10^-6cm³mol^-1, which is only marginally different from the value obtained for [OsCl(P N P^tBu)]. There-fore, it seems not to be possible to draw a simple, linear correlation between experimentally determinable TIP and ³¹P NMR shifts which one might be tempted to assume.

Nevertheless, 29 obviously features strong SOC effects. To back up these data, theoreti-cal theoreti-calculations were performed (see Part III, Section 3.4 for further details on the exact methods and settings used). To this end, the X-ray structure of 29 was used as a starting point and optimized for both the S=0 andS=1 state without any constraints. The derived PBE0/RIJCOSX/D3BJ/def2-TZVP||PBE/RI/D3BJ/def2-SVP energies indicated the S=1 state to be favorable by ∆G= −36.5 kJ mol^-1, which obviously does not reflect the experi-mental findings. Hence, higher level calculations were needed. The DFT-predicted minimal energy structure was the geometry used for all further calculations.

The SQUID data show 29 to exhibit electronic states which are rather close in energy to the ground state. Such quasi-degenerate systems are poorly described by single-reference methods like DFT or coupled cluster (CC). Therefore, complete active space self consistent field (CASSCF) calculations were performed on this system, as they are capable of describing strongly statically correlated systems and serve as starting point for multi-reference methods like N-electron valence state perturbation theory (NEVPT2) or quasi-degenerate perturbation

88 Chapter 3 Rhenium complexes ofiso-propyl based PNP pincer ligands for dinitrogen activation

theory (QDPT). Starting in the SVP basis, a CASSCF(14,10) calculation² was set up which included the five metal-centeredd-orbitals as well as the five ligand based MOs balancing the active space (i.e. the bonding orbitals corresponding to the antibondingd-space, see Figure 3.5). Inclusion of further, chloride centered orbitals, which are dominated by Clporbitals and are basically non-bonding, did not lead to significant variation of the results (no improvement of the overall energy, negligible influence on excited state energies). Preliminar results based on a reduced CASSCF(4,5) active space including only thedorbitals indicate that the overall picture is only slightly affected by the ligand based orbitals. However, having a balanced active space was considered good practice and since the (14,10) space was computationally perfectly feasible, this was used.

The final set of orbitals was obtained after reconverging the CASSCF(14,10) calculation in the TZVP basis averaging over a total of 50 singlet, 45 triplet and 5 quintet states (i.e.

over all possible excitations within the dorbitals) and the obtained energies were corrected for dynamic correlation by means of NEVPT2. The triplet ground state is confirmed with

2Throughout this thesis, for the active space of the performed calculations the following notation will be employed: method(nactive electrons,nactive orbitals).

MK

LOP

0

-2

-4

-6

-8

-10

-12 2

QU V W QU VX QU V Y

1.87 1.87 1.281.25

QU ZV

0.50 0.33

x,z(Re-Cl_equatorial+P)

x,y(Re-Cl_axial+P)

z(Re-Cl_equatorial)

y(Re-Cl_axial)

y(Re-Cl_equatorial) dyz

d_xy dxz

dz²

d_x2-y²

4

Fig. 3.5. MO scheme of29 from a stage-averaged CASSCF(14,10) calculation in the triple-ζ basis averaging over 50 singlet, 45 triplet and 5 quintet states. Occupation numbers (red), orbital labels (blue) and orbital plots at an isovalue of 0.04 are given as well.

3.1 The starting platform - [ReCl₃(HPNP^iPr)] (29) 89

Erel

Fig. 3.6. State-energy diagramm of 29 based on a NEVPT2/CASSCF(14,10) calculation. Non-relativistic states |S, MSi (blue) and their dominant configuration [XXXXX] with the re-spective weights in brackets (turquoise) are given (left), where each X corresponds to the occupation of the d-orbitals in energetically increasing order (the ligand orbitals are fullly occupied in all configurations and are thus omitted). Spin-orbit states derived from QDPT treatment (right) are color-coded according to the dominant spin-free state and individual contributions are given (weights≥10 %, on the dashed lines). State energies are given in cm^-1 respective to the ground states.

a (d_yz)², (d_xy)¹, (d_xz)¹ electron configuration (87 % weight), while the first exited triplet, which lies only 5.5 kJ mol^-1above the ground state is mainly composed of the corresponding (d_yz)¹, (d_xy)², (d_xz)¹ (87 % weight), reflecting the near-degeneracy of the d_xy and d_yz orbitals. However, if these states were subjected to a SOC calculation by means of a QDPT treatment, the degeneracy of each state is lifted. The lowest eigenstate is stabilized by

∆E_SOC =−3301 cm^-1 and features considerable multireference character, i.e. it is mainly composed from|1i(45 % weight), |1^′i(35 % weight) and0i(10 % weight) (see Figure 3.6).

The strong SOC effect, especially on the ground state are in good agreement with the ob-served TIP from magnetometry (see Figure 3.4) and simulated SQUID data from the QDPT calculation reproduce the TIP value exactly (XMT_QDPT = 1062.3·10^-6cm³mol^-1). The computed SOC stabilization energy is comparable to that of [OsCl(P N P^tBu)] (∆E_SOC=

−9.2 kcal mol^-1 ≈ −3200 cm^-1) and would therefore support a linear correlation of ∆ESOC

andXM(TIP). However, further data would be needed to underpin this suggestion.

Interestingly, the SOC calculation predicted significant oscillator strengths for the|0i → |4i and |0i → |5i electronic excitations, with very low excitation energies close to the near

90 Chapter 3 Rhenium complexes ofiso-propyl based PNP pincer ligands for dinitrogen activation

infrared (NIR)/mid infrared (MIR) limit (3614 cm^-1 and 3687 cm^-1, respectively) (see Part III, Table 3.12). And indeed, ATR-IR on a solid sample as well as regular transmission IR measurements in Nujol revealed a very prominent and broad peak at 3507 cm^-1with a small shoulder at slightly lower energies, which is in very good agreement with the calculations.

While the band shape and central frequency strongly reminds of water, this result was verified with different batches, each of which were pure by elemental analysis and showed no sign for water in the NMR spectra, excluding this to be the source of the signal. Additionally, and H/D exchange at the backbone amine by stirring with D₂O³ proved the smaller and sharper peak atν˜= 3181 cm^-1 to be the N H stretching vibration (ν

N D: ν˜= 2363 cm^-1;

∆˜ν_{harm. osz.} = 858 cm^-1).

IR measurements with an applied magnetic field and plotting of the obtained data as^TB/^TB=0

allowed to unambiguously identify the observed spectroscopic feature as an electronic ab-sorption, detectable with a regular MIR spectrometer (see Figure 3.7, left). All vibrational absorptions, which by nature are not influenced by an magnetic field, are canceled out in this spectrum and only electronic excitations which are perturbed by the field become visible.

However, the shift is so small that a quantification of the effect is difficult, which is in line with the simulations of the effect by QDPT calculations, where by applying a field along the y-axis a maximum shift of ≤ 1 cm^-1 at 10 T is predicted (see Figure 3.7 right and Part III Section 3.4.5).

Fig. 3.7. Left: Relative change of IR transmission in applied field, plotted as^TB/^TB=0. The original y-intercept (1) of each spectrum is manipulated so that the spectra appear stacked. Right:

QDPT simulated influence of a magnetic field on the energies of the states|4iand|5i.