The reaction of rhenium nitrosyl with a sterically hindered NHC-carbene

In this article, we present the serendipitous synthesis of the unknown Re(i) complex [(OPPh3)Re(NO)2Cl3] (3) that we obtained reacting the Re(v) complex trans-[(PPh3)2ReOCl3] (1) with NO gas in presence of CH3COOH. We found that 3 reacts with 1,3-bis (2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IMes) to yield a stable oximate—Re(iii) complex [(OPPh3)Re(NO)(ON 
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 IMes)Cl3] (4). We speculate that the IMes reacts with a bent NO, because the DFT calculations excluded the formation of both dimeric and η2–NO complexes in solution. The reactivity of the NO toward the carbene is probably due to an internal fluxional process in which the NO passes from linear to bent, triggered by the π-electrons given by the three chlorides to the Re through the mesomeric effect.


Introduction
Nitric oxide (NO) is an intriguing molecule that holds many roles, both in Biochemistry (as a bio regulator) 1,2 and Organometallic Chemistry (as non-innocent ligand). 3 It may possess a character of: cation (nitrosyl or nitrosonium, NO + : isoelectronic with CO and N 2 , hybridized sp, linear, 10 electrons, 1 electron donor), radical (nitric oxide, NO • , hybridized sp, linear, 11 electrons, 2 electrons donor) or anion (nitroxyl HNO, with its conjugate base NO − : isoelectronic with O 2 , hybridized sp 2 , bent, 12 electrons, 3 electrons donor). Depending on the conditions in which it is synthesized, the radical species can be oxidized to NO + or reduced to NO − or it may simply react as a radical (NO • ). It is actually possible to have another form of NO • , i.e., the dimeric one (NO) 2 2− . The latter dianion has been isolated for the first time by Evans and co-workers and it has a linear structure with a double bond between the two N [(ONvNO) 2− ]. 4 The ligands of the metal and its oxidation state dictate the binding mode of the NO. For instance, Keilwerth and Meyer presented a series of complexes in which Fe is bound to the tris-NHCs ligand tris[2-(3-mesitylimidazol-2-ylidene)ethyl]-amine (TIMEN Mes ). 5

Results and discussion
Re nitrosyls showed extremely good catalytic activity [11][12][13][14][15][16][17][18][19][20] and, as entry point to access the Re-NO complexes, we focused our attention on the well-known Re(V) trans-[(PPh 3 ) 2 ReOCl 3 ] (1), [21][22][23] complex that has shown to react with NO in benzene to give the paramagnetic complex Re(II) [(OPPh 3 ) 2 Re(NO)Cl 3 ] (2). 24 When we attempted the synthesis of complex 2 following a procedure similar to the one reported by Dziȩgielewski and co-workers (Scheme 1A), 24 we did get the wanted product 2 but together with the new dinitrosyl Re(I) diamagnetic complex [(OPPh 3 ) 2 Re(NO) 2 Cl 3 ] (3) (Scheme 1B) as confirmed by: FTIR (ESI, Tables S1 and S5 ‡), crystal structure ( Fig. 1) and HR-ESI MS analysis (S19, ESI ‡). The assignments of the NO of the FTIR will be discussed later in the paper. . All these different results could rise from one or more different factors as: type of solvent, amount of solvent used, amount of NO reacted, temperature, reaction time and presence of compounds able to react with NO giving rise even to more effective nitrosating agents. For instance, it is known that the solubility of the NO depends strongly from the type of solvent used in the reaction. We schematized below the differences in the reactions described above (Table 1). Furthermore, in the procedure followed by Abram, after that the flux of NO was stopped, it was added absolute EtOH and the solution was left overnight in contact with air. Neither in our synthesis (under N 2 ) nor in the case of the synthesis of Dziȩgielewski (under Ar) the contact with O 2 took place. 24 It is important to point out that NO reacts with O 2 yielding other two reactive species, i.e., the radical nitrogen dioxide ( • NO 2 ) and the dinitrogen trioxide (N 2 O 3 ). The latter has a structure similar to the Angeli's salt (Na 2 N 2 O 3 ) (Scheme 2). 26 The presence of all these different nitrosating agents together with O 2 , may explain why the complex [(PPh 3 ) 2 ReOCl 3 ] gave rise to many different products, including some that had as a counter ion an oxidized form of Re ([ReO 4 ] − ). Finally, we found that residual CH 3 COOH, present in the synthesis of 1, was responsible for the formation of 3. In

Paper Dalton Transactions
fact, it is known that CH 3 COOH is able to form nitrosyl carboxylate reacting with derivatives of the NO as the nitrosyl chloride (ClNO). 27 We tried to figure out the mechanisms of the formation of both the complexes 2 (Scheme 3A) and 3 (Scheme 3B), assuming that CH 3 COOH is present just in the reaction that yields 3. It is worth noting that the CH 3 COOH is needed in catalytic amount since during the reaction is not consumed. To confirm our findings, we synthesized a new batch of 1 without CH 3 COOH, and we obtained just 2. However, with 100 μL of CH 3 COOH for 40 mL of toluene and 0.85 g of the freshly synthesized 1 we obtained the mixture of 2 and 3, in less than 60 minutes at 80°C. We did try to improve the yield of the synthesis of 3 varying the amount of NO employed and the conditions of the reaction, but without success. The reaction always yielded a mixture of mono-and di-nitrosylated complexes and in some cases more than two products (ESI, S25 ‡). Then, we reacted 3 with 1,3-bis (2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IMes free carbene), to try to substitute the TPPO present in both 2 and 3.
The IMes reacted just with 3, but instead of replacing the TTPO as we expected, it reacted with one of the NOs coordinated to the Re yielding 4 (Scheme 4A). The latter possesses a carbene-oxime like ligand as proven by the crystal structure (Fig. 2). Instead, 2 is unreactive toward the carbene used in the reported conditions. We have drawn the complexes 5 and 6 that should have formed theoretically (Scheme 4B). To our best knowledge the only example reported of a triplet carbene that reacted with NO is the one reported by Forrester and Sadd that used NO to convert the carbene into an iminoxyl radical to observe the latter by E.S.R. spectroscopy. 28 Something similar was made by Cuesta and co-workers that synthesized the complexes [Re(ONCMe 2 )(CO) 3 29 However, in the latter case, the oxime group was introduced at once at the Re centre and it was not formed, as in our case, after the reaction of a NO with a carbene. We can say that our is the first example of this kind, nevertheless, why the carbene reacted with the N of the NO? Probably, the IMes acted as electrophile toward the N of a bent NO, during an ongoing equilibrium in which the NO passes from linear to bent. However, the carbene reacted in that way probably because of the presence of a NO in trans to the TPPO. The NO preferred to keep the latter instead of having the most trans labilizing carbene.  The theoretical reaction in which the carbene replaces the OPPh 3 (path B) to produce the theoretical complexes 5 and 6 and the reaction that instead took place ( path A). In the latter, the carbene attacked the N of a NO yielding 4.

Dalton Transactions Paper
This journal is © The Royal Society of Chemistry 2022 For instance, the work of Yang and co-workers can be an example of this strong trans labilizing effect of bulky carbenes. 30 We tried to obtain the corresponding labelled versions of the complexes 2 and 3 using 15 NO gas, reacting the latter with 1 in toluene in the presence of CH 3 COOH. The procedure for the evolution of 15

DFT calculations
Theoretical calculations have been carried out with the help of the Gaussian 16 program package 31 using the B3LYP functional 32 in conjunction with the Stuttgart/Dresden (SDD) effective core potential 33 for the metal centre and the standard 6-31+G(d) basis set 34 for the remaining atoms to investigate the possibility of the appearance of dimeric species formed from 3 after the loss of a TPPO in solution. Furthermore, other calculations were carried out to gain insight into the reaction between the carbene and one of the NOs of 3. The geometries were optimized starting from the structure of 3 determined by X-ray crystallography (Fig. 1). Through these calculations, we were able to exclude the formation of both η 2 -NO and dimeric structures, into which 3 could transform once in solution, after loss of the TPPO by dimerization (Schemes S23 and S24, ESI ‡). Once we understood, which structure 3 could have in solution, we investigated the thermodynamics of the reaction between the IMes free carbene and B1 (Scheme 5). We assumed that the IMes attacked the N NO atom directly after the NO bent and after this attack, the NO-IMes intermediate rearranged giving rise to 4. The calculations revealed that an alternative isomeric structure of 3, in which both NOs are in trans position to the Cl, could be involved (Scheme 5, B1). This isomer has a calculated energy that is slightly higher (+4.9 kcal mol −1 ) than the structure of 3 that is present in solid state. However, the attack of the IMes carbene can occur on the NO trans to the Cl, which again would give rise to an oximate-like ligand structure and this would lead to the formation of an isomer of 4 with the same calculated energy. The NO that reacts with the IMes, gets destabilized by the π-donor Cl. For instance, similar conclusions were drawn by Fomitchev and Coppens studying Ru(II) NO complexes with the NO in trans positions to ligands, such as: OH − , NH 3 , Cl − , SCN − , Br − , I − with different π-donor abilities. 35 They found that the higher is the π-donation of the ligand in trans to the NO, the weaker is the binding of the NO toward the metal. The order: OH − < NH 3 < Cl − < SCN − < Br − < I − of increasing donor ability was found. The OH − is an exception probably because of a strong σ (inductive) interaction that overcomes the π (mesomeric) one. 37 We can also see that the intermediate B2A has a lower energy (−4.2 kcal mol −1 ) when compared with the intermediate B1A, that would form from the isomer B1. However, at room temperature the pathway including B2 is the favoured one.

Intramolecular redox equilibrium of the complex mer-[(OPPh 3 )Re(NO) 2 Cl 3 ]
As we can see from the FTIR of the mixture of the complexes 2 and 3 (Table 1), there are two strong peaks at 1391 and 1378 cm −1 . We assigned these two peaks to the NO belonging to 3. We did it because: the FTIR of 2 alone does not contain such peaks, the FTIR of the labelled complexes 7 and 8 contains the same peaks. The latter disappears after the reaction of the mixture of 2 and 3 with the IMes (ESI, S3-S5 and S16 ‡). We believe that these two peaks are the result of a fluxional process that involves the uneven distribution of the π* orbital between N and O of the NO. In fact, the behaviour of the NO depends on the amount of charge that lies on these atoms.

Paper Dalton Transactions
Usually, the π* orbital is localized as 60% on the N and 40% on the O and the localization of such orbital influences the geometries (M-NO, M-ON, M-η 2 -NO, …) of the M-NO complexes. 36 Moreover, the π* orbital works as an acceptor orbital that NO can exploit to accept electronic charge from the metal. In our case the distribution of the π* orbital of the NOs of 3 probably is such to create, for the M-NO bond of both NOs, an internal equilibrium between a Re-nitrosonium NO + and a Re-nitroxyl NO − . This equilibrium would account for both the multiple signals in the 15 38 The authors speculated that different types of Co-NO bonding were in equilibrium, generating in that way two different ν NO bands. It is possible as well to have a dinitrosyl complex with both linear and bent coordinated NO groups present at the same time. [39][40][41][42] Considering the existing literature, we believe that the complex mer-[(OPPh 3 )Re(NO) 2 Cl 3 ] probably possesses an internal fluxional process that is responsible for the reactivity of its NOs and for all the signals present in the 15 43 In the complex 3 all the d orbitals d x 2 -y 2 and d z 2 available for a bond were already full and there were no other free orbitals with proper geometry that could have been used to accommodate the π-electrons. Then, the presence of these π-electrons created a destabilization named "filled-filled" interaction. The two NOs bound to the Re, being π-acid ligands, helped the metal reducing the filled-filled interaction through a push-pull mechanism. The NOs did that accepting the electrons coming from the d xz metal orbital in their π*orbital. 44 As anticipated above, we believe that in our case the extent of the electron-transfer was such that the NO got a nega-tive charge forming species as for instance the Re(III) [(OPPh 3 )Re (NO + )(NO − )(Cl) 3 ] that are in a fluxional equilibrium with the Re(I) [(OPPh 3 )Re(NO) 2 (Cl) 3 ]. The reactivity of the N holding the negative charge toward the IMes was then higher than in a linear NO. This particularly would account for the unique reaction pattern that the NO and the IMes followed and that yielded 4.

Conclusions
In this paper, we presented the reaction of 1 with NO gas that in the presence of CH 3 COOH brings to the serendipitous synthesis of the unknown complex 3 together with the expected complex 2. Then, we found that 3 reacts with a bulky carbene (IMes) to yield the stable oximate-Re(III) complex 4, while 2 is inert toward the carbene in the same reaction conditions. Since the DFT calculations excluded the formation in solution of both dimeric NO complexes and η 2 NO complexes, we concluded that the IMes reacted with a bent NO. The reactivity of the NO toward the IMes is probably due to an internal fluxional process in which the NO passes continuously from linear to bent, triggered by the π-electrons given by the three Cl to the Re through the mesomeric effect.

Author contributions
OB performed the X-ray diffraction analyses, the structure determinations and run the DFT calculations. GG conceived the project, synthesized all the complexes cited in this work and crystalized them, performed all the experiments, wrote the original draft, reviewed and edited it.

Conflicts of interest
There are no conflicts to declare.