Persistent four-coordinate iron-centered radical stabilized by π-donation

Generation of four-coordinate iron-centered radical 3 was realized by the thermal homolysis of the unsupported Fe–Fe bond of 2.


Introduction
The diverse reactivity of transition metal-centered radicals towards various organic substrates has garnered signicant interest in both organometallic and catalytic synthetic chemistry over the past several decades. 1,2 Among the transition metal complexes, iron carbonyl based ve coordinate iron(I) species such as A-C shown in Chart 1 have received particular attention owing to their catalytic activity and unique properties. 3-6 Thus, construction of coordinatively unsaturated iron(I) carbonyl complexes is expected to facilitate the development of novel organometallic and catalytic reactions. However, few examples of four or less coordinate iron(I) carbonyl complexes have been explored. [7][8][9] Holland et al. described the synthesis of a four-coordinate Fe(I) dicarbonyl complex D in which the iron center adopts an S ¼ 1/2 electronic conguration. 8 Parkin et al. synthesized a four-coordinate Fe(I) monocarbonyl complex E using a tris(pyrazolyl)borate ligand. 9 Chelating multidentate ligands bearing sterically hindered substituents are needed to stabilize these coordinatively unsaturated species. ‡ We have attempted to construct a reactive, coordinatively unsaturated, iron carbonyl based iron-centered radical based on an alternative synthetic strategy involving stabilization by ligand-to-metal p-donation. Phosphinyl radical 1, 10 shown in Scheme 1, was thought to be appropriate for this purpose, as it exhibits reactivity towards transition metal complexes, and the lone pair of electrons on the phosphorus center can effectively stabilize the low-coordinate metal complex via p-donation. 10b In this paper, we report that 1 functions as an effective ligand to form the dinuclear iron carbonyl complex 2 with an unsupported Fe-Fe bond. Complex 2 was a suitable precursor for the generation of four-coordinate iron-centered radical 3 via homolysis of the Fe-Fe bond. Reactions of 3 with organic radicals are also reported.
A yellow suspension of Fe 2 (CO) 9 in pentane gradually dissolved to give a dark red solution during the course of the reaction with 1 at room temperature for 16 h. From the dark red solution, complex 2 was obtained in 86% yield (based on 1) (Scheme 1). The reaction was accompanied by the formation of Fe(CO) 5 , which was conrmed by IR spectroscopy. In the course of the reaction, radical 1 formally underwent a one-electron reduction to form a monoanionic phosphido ligand. As a closely related example to this reaction, Cowley et al. prepared a vecoordinate iron-centered radical via the reaction between Fe 2 (CO) 9 and [{(Me 3 Si) 2 CH} 2 P]c. However, the molecular structure of this complex was not determined crystallographically. 11 Complex 2 consists of two trigonal-bipyramidal iron centers with six CO ligands (Fig. 1). The distances between the iron and the carbon atoms of the CO ligands coordinated to the other iron center were within 2.932(4)-3.825 (6) (tim ¼ 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) 12a and [(3,5-i Pr 2 -Ar*)Fe-FeCp(CO) 2 ] ((3,5-i Pr 2 -Ar*) ¼ C 6 H-2,6-(C 6 H 2 -2,4,6-i Pr 3 ) 2 -3,5-i Pr 2 -Ar*). 12b The Fe-Fe bond distance in these complexes (2.6869(6) 12a and 2.3931(8) 12bÅ ) are signicantly shorter than that of 2 (2.7374(10)Å), which reects the weak bonding interaction between the two iron centers in 2 (vide infra). The sum of the three angles around P(1) and P(2) are 358.6 and 358.1 , respectively, indicating planarity at P, which is consistent with p-bonding. The short Fe-P bond lengths, 2.0934(12)Å for Fe-P(1) and 2.1047(13)Å for Fe(2)-P(2), also support the multiple bond character of the Fe-P bond (vide infra). 14 Complex 2 is diamagnetic, and the 1 H NMR of 2 conrms the molecular structure. The formal oxidation state of the two iron centers in 2 can be considered as Fe(I), and an intramolecular antiferromagnetic coupling between two adjacent iron centers makes complex 2 diamagnetic. A sharp singlet appears at 0.35 ppm in toluene-d 8 at 293 K, whereas four slightly broad singlets are observed at 0.31, 0.35, 0.37, and 0.48 ppm with an integral ratio of 1 : 1 : 1 : 1 at 193 K. These signals are attributed to the magnetically inequivalent SiMe 3 moieties in 2. Notably, the 31 P NMR spectrum shows one signicantly downeld shied peak at 425.9 ppm, which is typically seen for planar sp 2 -phosphorus species. 15 This 31 P peak, as well as the planar geometry around P and the short Fe-P bond distances observed in X-ray crystallography, strongly suggest that the phosphido ligands formally function as LX-type ligands with the aid of p-donation.
At higher temperatures (293-353 K), one sharp singlet for the SiMe 3 group and one doublet due to the CH 2 moiety of the backbone of the phosphido ligand are observed in the 1 H NMR of 2. Additionally, it should be noted that the appearance of a broad signal is conrmed at $3-5 ppm. The intensity of this broad peak increases with temperature ( Fig. S5-2 in ESI †). Considering the elongated Fe-Fe bond in 2, we hypothesized that the broad peak observed at higher temperatures could be ascribed to the generation of paramagnetic species 3 via the homolytic cleavage of the Fe-Fe bond, as shown in Scheme 2. The generation of a radical species is supported by ESR spectroscopy; the ESR spectrum of 2 in toluene exhibits one broad signal at g ¼ 2.0519 (A( 31 P) ¼ 3.43 mT) at 293 K. The intensity of this peak gradually increases upon heating, indicating that the concentration of the ESR-active species signicantly increases with temperature ( Fig. S6-1 †). The observed g-value is comparable to those reported for iron carbonyl-based ve-coordinate seventeen electron radicals. 1a,3b,3c In addition, the ESR spectrum of a ash-frozen toluene solution measured at 77 K showed a rhombic signal (g ¼ 2.0838, 2.0573, 2.0409), suggesting that the iron species generated in situ has an S ¼ 1/2 ground state ( Fig. S6-2 †).
IR spectroscopy provided more detailed information about the species generated at higher temperatures. The solid-state IR (ATR) spectrum of 2 features bands at 2014, 1963, 1932, and 1916 cm À1 , which are attributed to terminal CO ligands ( Fig. S7-1 †). In the IR spectrum of 2 in n-octane at 293 K, four n CO absorptions appear at 2017, 1968, 1940, and 1921 cm À1 , indicating the presence of dinuclear complex 2 in solution at this temperature (Fig. 2). An absorption band assignable to the bridging CO ligand is not observed in the solid or solution states. The solution spectrum at 293 K includes a shoulder at around 1938 cm À1 together with the four aforementioned n CO absorption bands. At 353 K in n-octane in the dark, this band represents the major absorption and is accompanied by a strong band at 2015 cm À1 , and the four CO bands observed at 293 K are almost completely absent (Fig. 2). If we assume that Scheme 1 Synthesis of dinuclear iron carbonyl complex 2. Fig. 1 Molecular structure of 2 with 50% probability ellipsoids.
three CO ligands are maintained on the iron center during heating, this spectral change could be explained by the formation of an iron tricarbonyl species with C 3v symmetry (vide infra).
The UV-Vis-NIR spectrum of an n-octane solution of 2 at 293 K displays an intense band (A) at 380 nm and two relatively weak bands (B and C) at 502 and 720 nm, respectively (Fig. 3,upper). Their intensity progressively decreases with heating, and three new bands, namely A 0 (362 nm), B 0 (496 nm), and C 0 (818 nm) are exclusively observed at 353 K (Fig. 3, lower). Aer cooling the solution to 293 K, the original bands, A, B, and C, gradually reappear over time, and equilibrate aer 5 h. Notably, an isosbestic point is observed at 810 nm, providing good evidence for an equilibrium between two species .
In order to gain further insight into the electronic structure and bonding in the iron complexes, a theoretical investigation of 2 was undertaken. Geometry optimization and time-dependent density functional theory (TD-DFT) spectral calculation were carried out at the PBE0, CAM-B3LYP, and B3LYP levels, and the PBE0 functional was found to give the most satisfactory results. § The calculated spectrum of 2 opt is given in Fig. 3 (upper). Although a low energy absorption band is predicted to appear at 670 nm, which is blue-shied by 50 nm as compared to the actual spectrum presumably due to the overestimation of the low-energy gap, the TD-DFT analysis of 2 opt is in reasonable agreement with the actual spectrum. The Mayer Fe-Fe bond order is calculated to be 0.33. The bond order between Fe and P is calculated to be 1.4, indicating this bond to have a double bond nature. Molecular orbital analysis of HOMO and HOMO À 1 of 2 opt highlights the p-bonding nature of the Fe-P bond (Fig. 4). This p-bonding nature was also conrmed by density distribution analysis in HOMO and HOMO À 1 (Fig. S17 †).
Next, we calculated the molecular and electronic structures of 3 at the PBE0 level. We found that 3 opt possesses a slightly distorted C 3v tetrahedral coordination geometry in the openshell doublet ground state (Fig. 5). Interestingly, DFT studies of 3 opt indicate that the spin densities around Fe and P are 1.50 and À0.52, respectively. The bonding orbital in 3 was carefully analysed, and the orbital interactions in 3 opt were depicted in Fig. S20. † We found that bHOMO consists of the d yz orbital in iron and p y orbital in phosphorus, and p-bonding character was conrmed in the bHOMO by molecular orbital analysis of 3 opt (Fig. 6). In contrast, aHOMO consists of d x 2 Ày 2 orbital in iron leading to a non-bonding interaction (molecular orbital details are given in the ESI, Fig. S19 and S20 †). This non-bonding nature was also conrmed by density distribution in aHOMO (Fig. S21 †). This partial Fe-P p-bonding is consistent with the estimated Mayer bond order for Fe-P (1.2). These results indicate that electron donation from the P atom to Fe found in the bHOMO electronically stabilizes the four-coordinate iron-   centered radical. TD-DFT calculations based on 3 opt give the absorption bands shown in Fig. 3 (lower). Although the accuracy of the TD-DFT calculation of 3 opt appears to be limited due to the open-shell nature of 3 opt , 16 the obtained predicted spectrum roughly approximates the actual spectrum. The two remarkable peaks in the range of 300-550 nm in the predicted spectrum of 3 opt are blue-shied compared with those of 2 opt . A similar feature is also observed in the actual spectrum of 3. It should be noted that one peak appears at 839 nm in the predicted spectrum of 3 opt , which effectively reproduces the actual band observed at 818 nm (band C 0 in Fig. 3 (lower)). The electron density difference map of 2 opt and 3 opt indicates that these absorptions originate from the metal-to-ligand charge transfer (MLCT) transitions ( Fig. S22 and S23 †). It should be mentioned that Lappert et al. have reported the synthesis of four coordinated Co(I) tricarbonyl, [{(Me 3 Si) 2 N}( i Pr 2 N)P] Co(CO) 3 , by the reaction of the in situ generated phosphinyl radical [{(Me 3 Si) 2 N}( i Pr 2 N)P]c with Co 2 (CO) 8 . 17 This cobalt(I) phosphido complex also adopts tetrahedral coordination geometry, as shown by X-ray diffraction analysis. Thus, complex 3 can be considered analogous to Lappert's cobalt(I) phosphido complex minus one electron.
Considering the experimental and theoretical results, it is highly probable that the generation of four-coordinate ironcentered radical 3 takes place via the homolysis of the unsupported Fe-Fe bond of 2 in solution. The coordination geometry of 3 was predicted to be slightly distorted tetrahedral, and the local symmetry to resemble C 3v . This is consistent with the IR spectrum of 2 in n-octane at high temperatures. As mentioned above, two strong bands are observed at 2015 and 1938 cm À1 at 353 K. This spectral feature is characteristic of tetrahedral C 3v -symmetrical tricarbonyl species; the former can be assigned as the symmetric A 1 stretch, and the latter can be assigned as the asymmetric E stretch. These CO stretching vibrations are blue-shied relative to those found in Holland's four coordinate Fe(I) dicarbonyl complex, (NacNac tBu )Fe(CO) 2 (1992 and 1908 cm À1 ), 8a but are signicantly shied to lower wavenumbers compared with those of previously reported ve coordinate Fe(I)-tricarbonyl complexes. 6a, 8a,8b The equilibrium constant (K eq ) between 2 and 3 was determined by variable temperature NMR spectroscopy. The concentration of 3 in solution was estimated by the Evans method, 18 and the thermodynamic parameters for the homolytic cleavage of the Fe-Fe bond in 2 were calculated to be DH ¼ 18.8 AE 0.5 kJ mol À1 and DS ¼ 46.1 AE 1.8 J mol À1 K À1 , respectively. The thermodynamic parameters were also estimated based on the concentration of 2 (DH ¼ 15.8 AE 0.2 kJ mol À1 and DS ¼ 36.7 AE 0.4 J mol À1 K À1 ), which are roughly consistent with those obtained by the Evans method.{ The large positive DS value is reasonable for the homolysis of Fe-Fe bond.
It is well known that metalloradicals can be captured by reaction with radical traps, such as stable organic radicals, metal-hydride species including HSnR 3 , and group 15 based cage compounds (e.g., P 4 ). As a recent representative example, Scheer et al. have described that the sterically hindered iron dimer [Cp BIG Fe(CO) 2 ] 2 (Cp BIG ¼ C 5 (4-nBuC 6 H 4 ) 5 ), readily dissociates in solution to afford a monomeric iron-centered radical, which can be easily trapped by reaction with P 4 or As 4 . 19 In manganese chemistry, Figueroa has synthesized an isolable mononuclear manganese-centered radical by introduction of sterically bulky isocyanide ligands, the reactivity of which was clearly indicated by its reaction with HSnR 3 and some organic substrates. 6k Because 3 consists of only one bulky phosphido and three less hindered CO ligands, the iron center of 3 is expected to have enough room to react with certain organic substrates. Thus, we performed reactions of 3 with organic radicals. First, treatment of 3 (generated in situ from 2) with 9azanoradamantane N-oxyl (nor-AZADO) was examined. The  Scheme 3 Reactions of in situ generated 3 with organic radicals. reaction proceeds smoothly, even at room temperature, to afford diamagnetic ve-coordinate mononuclear iron(II) complex 4 in 74% isolated yield (Scheme 3).k Identication of 4 was carried out by IR and NMR spectroscopy, and X-ray diffraction analysis. The signals due to the SiMe 3 groups of 4 appear as two singlets at 0.36 and 0.47 ppm in the 1 H NMR spectrum in C 6 D 6 . One phosphorus resonance is observed at 282.16 ppm in the 31 P NMR spectrum. It is shied to a higher eld as compared to 1, presumably due to the coordination of one s-donating nitrogen instead of a p-accepting CO ligand. Two strong absorption bands derived from the Fe-CO moiety appear at 1958 and 1892 cm À1 in the IR spectrum of 4. The ORTEP representation of 4 is depicted in Fig. 7. Two CO ligands are bound to the iron center, and the O-N bond of nor-AZADO is coordinated to the iron center in a k 2 -fashion with a bond distance of 1.3851 (14)Å. The sum of the three angles around P is 358.1 , indicating the contribution of p-bonding. Although 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and its analogues are known to coordinate to the metal center in a k 2 -(ON) coordination mode in the reaction with some transition metal complexes, 20 to the best of our knowledge, complex 4 is the rst example of a structurally characterized nor-AZADO complex having a k 2 -(ON) moiety. The overall reaction can be explained as follows: radical recombination between in situ-generated iron-centered radical 3 and the oxygen-centered radical takes place to create the Fe-O bond, followed by the liberation of one CO ligand from the iron center concomitant with the formation of the Fe-N bond. In the course of this reaction, nor-AZADO formally undergoes one electron reduction to coordinate to the iron(II) center.
Next, we performed the reaction between 3 and phosphinyl radical 1, from which formation of ve-coordinate iron-silyl complex 5 and phosphaalkene 6 in a 1 : 1 ratio is conrmed. The presence of p-donation in 5 was ascertained by 31 P NMR, as one signicantly downeld shied singlet is observed at 461.4 ppm. The molecular structure of 5 was unequivocally determined by X-ray diffraction analysis (Fig. S27 †). The relatively short Fe-P bond length (2.1009(7)Å) as well as the planar geometry around the P atom (the sum of the three angles around P was found to be 359.92 ) indicate the contribution of p-donation to the Fe-P interaction. This reaction can also be considered the result of the radical behaviour of complex 3; the iron-centered radical 3 abstracts one of four SiMe 3 groups from 1 via homolytic substitution (S H 2) to form a Fe-SiMe 3 bond concomitant with the formation of the C]P bond to afford 6 (detailed reaction mechanism is shown in Scheme S1 in ESI †). Given the fact that diamagnetic closed shell organoiron(II) complexes tend to adopt coordinatively saturated octahedral coordination geometry, the results described here could contribute to the development of new synthetic methodologies towards novel iron complexes.
Reactions of in situ generated 3 with HSnR 3 (R ¼ Ph, Bu) were also performed. Reaction of 2 with HSnPh 3 was monitored by 1 H and 31 P NMR spectra in C 6 D 6 at room temperature. In the 31 P NMR spectrum of the crude product, three peaks were observed at À33.6, 29.3 and 464.9 ppm, respectively, in an intensity ratio of 10 : 4 : 1. The peak at À33.6 ppm was assigned as free phosphine 7, 10a whereas the other two peaks can be assigned as complexes 8 and 9. Although 9 could not be obtained in pure form (see ESI † for detail), complex 8 can be isolated in 26% yield, and the molecular structure of 8 was determined by X-ray diffraction analysis (Fig. S28 †). The relative ratio of 7, 8 and 9 was determined from the 1 H NMR spectrum of the crude product using an internal standard, and was estimated to be 60 : 37 : 3 based on iron. Formation of trace amount of Ph 3 Sn-SnPh 3 (ca. 2%) was detected in the 1 H NMR spectrum of the crude product.** These results strongly suggest that the reaction of 3 with HSnPh 3 gave a complex mixture, and the major reaction pathway is the formation of free phosphine 7. At the rst stage of this reaction, iron-centered radical 3 would abstract a H atom from HSnPh 3 to generate ve coordinate "(phosphido)Fe(H)(CO) 3 " species. Because such Fe-H species is expected to be unstable, following disproportionation would afford 7 and iron species including 8 and 9. This possible reaction sequence is shown in Scheme S2 in ESI. † In a similar manner, reaction of 2 with HSnBu 3 was also performed. The 31 P NMR spectrum of the crude product suggests the formation of 7 as the major product concomitant with the formation of 8 0 which is analogous to 8. The slightly broadened 1 H NMR signals are presumably due to the partial formation of paramagnetic species and prevented further detailed analysis. Formation of 7 was also conrmed in the crude 1 H NMR spectrum (Fig. S13 †). Formation of Bu 3 Sn-SnBu 3 was observed in ca. 10% yield in the GC-MS analysis, suggesting that the partial radical recombination took place aer the abstraction of a H atom from the Sn center (Scheme 4). The reactions shown above clearly indicate the radical nature of 3. Next, we have performed the reaction of 3 (generated in situ from 2) with 9,10-dihydroanthracene or 1,4-cyclohexadiene. However, no reaction took place below 333 K, and partial decomposition of 2 was conrmed by 1 H and 31 P NMR in the reaction at 353 K for 24 h. Considering the difference in the bond dissociation energy for the C-H bond of 1,4-cyclohexadiene (73 kcal mol À1 ) 21a and the Fe-H bond (58.5 AE 5 kcal mol À1 ; determined in the gas phase), 21b the results obtained here may be reasonable. Further research to elucidate the bond dissociation energy of Fe-H derived from complex 3 will be carried out in our laboratory.

Conclusions
In conclusion, we succeeded in preparing coordinatively unsaturated dinuclear iron carbonyl complex 2 by the reaction of Fe 2 (CO) 9 with phosphinyl radical 1. The generation of fourcoordinate iron-centered radical 3 was realized by the thermal homolysis of the unsupported Fe-Fe bond of 2. Experimental analysis and theoretical calculation revealed that p-donation from the phosphido ligand to the iron center electronically stabilizes the four-coordinate iron-centered radical 3. In both complex 2 and 3, p-bonding electrons in the p-orbital of phosphorus lies on the HOMO and HOMO À 1 for 2 and bHOMO for 3, which are effectively involved in p-donation from phosphorus to the iron center. The orbital interactions in 3 opt shown in Fig. S20 † clearly suggest that the 3p y orbital of phosphorus and the d yz orbital of iron can effectively interact to create the pbonding interaction. Complex 3 effectively reacted with organic radicals to afford diamagnetic ve-coordinate organoiron(II) species. These results may facilitate the development of new synthetic methodologies towards the design and construction of highly reactive metal-centered radicals. Efforts to develop novel fundamental and catalytic reactions realized by coordinatively unsaturated iron-centered radicals are now underway in our laboratory.
Notes and references ‡ Although complex D in Chart 1 can capture atmospheric CO, no further reactions of four coordinated iron carbonyl based Fe(I) complexes have been reported. § The ground state of 2 opt was estimated to be the closed-shell singlet in all calculations. Calculated bond lengths and angles at the PBE0 and CAM-B3LYP levels are similar to experimental values; however, the structural parameters at the B3LYP level are inconsistent with the experimental results (see Table S2 in ESI †). The results of TD-DFT calculations at the CAM-B3LYP and B3LYP levels are also reported in ESI (Fig. S24 †). { The thermodynamic parameters were calculated based on the concentration of 3, which was estimated by the Evans method, 18 in which the number of unpaired electrons at the iron center was assumed to be 1. Because it is difficult to quantitate the exact concentration of 2 and 3 simultaneously in solution, the thermodynamic parameters were alternatively calculated based on the concentration of 2, as estimated from the relative ratio of the integral value in the 1 H NMR spectra measured in C 6 D 6 at various temperatures. Although the relative ratio of the integral values of sharp (diamagnetic) vs. broad (paramagnetic) peaks may include some experimental error, the obtained thermodynamic parameters were roughly consistent with those obtained by the Evans method. The details are given in ESI. † k One of the reviewers pointed out that dinuclear complex 2 may have the possibility of directly reacting with nor-AZADO to form product 4. Because one broad peak assignable to 3 almost disappeared in 1 H NMR spectrum of 2 at 253 K, we have performed the reaction of 2 with nor-AZADO in toluene-d 8 at 253 K. We found that only ca. 8% of 2 was converted to product 4 aer 25 days at 252 K. This result may indicate that in situ generated radical 3 can react with nor-AZADO, and that direct reaction of dinuclear complex with nor-AZADO can be ruled out. ** Although powder formation was not observed aer the reaction of 2 with HSnPh 3 , low solubility of Ph 3 Sn-SnPh 3 toward C 6 D 6 may hamper the accurate detection of the amount of Ph 3 Sn-SnPh 3 formed in this reaction.