English

• 1.   Introduction

  In the reactions of metal species with diazoalkanes, diazoalkane complexes^[1-5] are recognized as intermediates en route to the products, alkylidene complexes, that could facilitate useful transformations such as alkene cyclopropa- nation, C—H alkylation, alkene metathesis, and so on.^[6-10] Consequently, there are great interests on the sythesis, structural feature, and reactivity of transition-metal diazoalkane complexes. The known transition-metal diazo- alkane complexes are dominated by the complexes of cobalt, nickel, and ruthenium, ^[11-16] whereas the diazoalkane complexes of iron are scarce, which form a sharp contrast to the soaring research interest on iron-catalyzed carbene- transfter reactions.^[17]

  The limited examples of iron diazoalkane complexes include the bis(imino)pyridine complex [(PDI)Fe(η¹-N₂CH-SiMe₃)] (A in Chart 1), ^[18] the tris(pyrrolido)ethane iron complex [(tpe)Fe(η¹-N₂CPh₂)][Li(THF)₄] (tpe＝tris(5-me- sitylpyrrolyl)ethane, B), ^[19] the β-diketiminate iron complex [L^Bu-tFe(η¹-N₂CPh₂)] (L^Bu-t＝[(DippNCBu^t)₂CH]^-, C)^[20], the cyclopentadienyliron complexes [(η⁵-C₅H₅) Fe(η¹-N₂C-Ar¹Ar²)(P-P)]BPh₄ (P-P＝1, 2-bis(diphenylphosphine)etha- ne, 1, 3-bis(diphenylphosphine)propane; Ar¹＝Ar²＝Ph; Ar¹＝Ph, Ar²＝p-tolyl; Ar¹Ar²＝C₁₂H₈, D), ^[21] as well as the side-on type diazo complexes [(PDI)Fe(η²-N, N-N₂C-Ph₂)] (E)^[22] and [L^Bu-tFe(η²-N, N-N₂CPh₂)K(OEt₂)]₂ (F)^[20]. Interestingly, spectroscopic and theoretical studies revealed that the diazo ligands in these complexes could be present in different electronic structures. For example, the diazoalkane ligands in B and C have been assigned as radical anions. The side-on diazoalkane ligand in E is thought to receive little backdonation from its iron center and can be viewed as merely a σ-donor. The side-on diazoalkane ligand in F on the other hand is a radical anion. In addition to the aforementioned studies, herein, we wish to report the reactions of high-spin iron(II) complexes featuring tripodal amido-phosphine-amido ligands, from which two new iron diazoalkane complexes [(κ³-N, N, P-^MesN₂P^Bu-t)Fe(η²-N, N-(p-tol-yl)₂CN₂)] and [(κ³-N, N, P-^MesN₂P^Bu-t)Fe(κ²-N, O-PhC(N₂)-CO₂Et)] have been isolated and structurally characterized.

  2.   Results and discussion

  2.1   Syntheses and characterization of (amido-phosphine-amido)iron(II) complexes 1 and 2

  Previously, we found that the reaction of an (amido-phosphine-amido)iron(II) complex [(κ³-N, N, P-^dfpN₂P^Ph)Fe-(THF)₂] with (p-tolyl)₂CN₂ furnished a diamido-phosphine ylide iron(II) complex [(κ³-N, N, C-^dfpN₂P^PhC(tolyl-p)₂)-Fe(THF)] (Scheme 1).^[23] The ylide complex was proposed to be formed from the migratory insertion reaction of an iron(II) alkylidene intermediate. Aiming to attenuate the ease of the migratory insertion reaction of the proposed iron(II) alkylidene intermediates, new amido-phosphine- amido ligands having tert-butyl on phosphorus atom were then developed.

  Figure Chart 1

  

  Figure Chart 1.  Examples of isolable iron diazoalkane complexes

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  Scheme 1

  

  Scheme 1.  Reaction of [(κ³-N, N, P-^dfpN₂P^Ph)Fe(THF)₂] with (p-tolyl)₂CN₂

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  The synthetic procedures of the new ligands (o-(N-(2, 4, 6-Me₃C₆H₂)NH)C₆H₄)₂PBu^t and (o-(N-(2, 6-Cl₂C₆H₃)NH)-C₆H₄)₂PBu^t (denoted as H₂(^MesN₂P^Bu-t) and H₂(^dcpN₂-P^Bu-t)) follow that used for H₂(^dfpN₂P^Ph).^[23] Treatment of Bu^tPCl₂ with 2 equiv. of (2, 4, 6-Me₃C₆H₂)(2-LiC₆H₄)NLi and (2, 6- Cl₂C₆H₃)(2-LiC₆H₄)NLi, which were generated in situ by the interaction of (2, 4, 6-Me₃C₆H₂)(2-BrC₆H₄)NH and (2, 6- Cl₂C₆H₃)(2-BrC₆H₄)NH with 2 equiv. of BuⁿLi, followed by quenching with water, afforded the amine-phosphine-amine compounds H₂(^MesN₂P^Bu-t) and H₂(^dcpN₂P^Bu-t) in 50% and 53% isolated yields, respectively (Scheme 2). H₂(^MesN₂-P^Bu-t) and H₂(^dcpN₂P^Bu-t) have been characterized by NMR, FT-IR and HRMS.

  Scheme 2

  

  Scheme 2.  Synthetic route to the (amido-phosphine-amido)iron(II) complexes 1 and 2

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  The new amine-phosphine-amine ligands H₂(^MesN₂P^Bu-t) and H₂(^dcpN₂P^Bu-t) readily undergo amine-elimination reaction with [Fe(N(SiMe₃)₂)₂]₂ (0.5 euiqv.) at room temperature to afford (amido-phosphine-amido)iron(II) complexes [(κ³-N, N, P-^MesN₂P^Bu-t)Fe(OEt₂)] (1) and [(κ³-N, N, P-^dcpN₂-P^Bu-t)Fe(OEt₂)] (2) in 49% and 77% isolated yields, respectively (Scheme 2). Complexes 1 and 2 are sensitive to air and moisture. They are soluble in diethyl ether, toluene and tetrahydrofuran, and have been characterized by ¹H NMR, solution magnetic moment measurement, ⁵⁷Fe Mössbauer spectroscopy, as well as elemental analysis. Their measured solution magnetic moments (μ_eff＝4.7(1)μ_B and 5.0(1)μ_B in C₆D₆ at room temperature for 1 and 2, respectively) and ⁵⁷Fe Mössbauer spectroscopic data (isomer shift δ＝0.75 mm/s, quadrupole splitting |ΔE_Q|＝1.19 mm/s for 1; δ＝0.82 mm/s, |ΔE_Q|＝2.40 mm/s for 2) are indicative of their high-spin iron(II) nature (S＝2). The difference in the ⁵⁷Fe Mössbauer data of 1 and 2 should be caused by their different N-aryl substituents.

  The molecular structure of 2 has been confirmed by single-crystal X-ray diffraction study. As shown in Figure 1, the amido-phosphine-amido ligand in 2 is coordinating with the iron center in a tripodal conformation, being similar to the reported (amido-phosphine-amido)iron(II) complex [(κ³-N, N, P-^MesN₂P^Ph)Fe(OEt₂)].^[24] Notably, while the Fe—N distances in 2 are comparable to those of their congeners in [(κ³-N, N, P-^MesN₂P^Ph)Fe(OEt₂)]^[24], its Fe—P bond with a distance of 0.24116(5) nm is longer than that in [(κ³-N, N, P-^MesN₂P^Ph)Fe(OEt₂)]^[24] [0.2367(1) nm]. The long Fe—P distance in 2 reflects the influence of the steric demanding nature of tert-butyl versus phenyl group. Another worth mentioning structural feature of 2 is its Fe—Cl(3) distance of 0.2845 nm, which lies between the sum of the two van der Waals radii (0.307 nm) and the Fe(II)—Cl distance in the five-coordinate (bis(imino)pyridine)iron(II) chloride complex [(PDI)FeCl₂]^[25] (0.2301 nm), indicating weaker interaction between the two atoms.

  Figure 1

  

  Figure 1.  Molecular structure of 2 (hydrogen atoms were omitted)

  Selected distances (nm) and angles (°): Fe1—N1, 0.19877(14); Fe1—N2, 0.19926(14); Fe1—P1, 0.24116(5); Fe1—Cl1, 0.2845; Fe1—O1, 0.20626(13); N1—Fe1—N2, 123.31(6); N1—Fe1—P1, 80.99(4); N1—Fe1—O1, 113.78(6); N2—Fe1—P1, 84.25(4); N2—Fe1—O1, 121.62(6); P1— Fe1—O1, 117.17(4); P1—Fe1—Cl1, 153.00

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  2.2   Reactions of 1 and 2 with diazoalkanes

  With the new (amido-phosphine-amido)iron (II) com- pounds 1 and 2 in hand, we further studied their reactions with (p-tolyl)₂CN₂, PhC(N₂)CO₂Et, and DmpC(N₂)H (Dmp＝2, 6-dimesitylphenyl), which were found to give different outcomes.

  2.3   Reaction of 1 with (p-Tolyl)₂CN₂

  The interaction of 1 with (p-tolyl)₂CN₂ (1 equiv.) in toluene or THF at room temperature produced a brown solution without N₂-evolution. Further workup and recrys-tallization led to the isolation of a diazoalkane complex [(κ³-N, N, P-^MesN₂P^Bu-t)Fe(η²-N, N-(p-tolyl)₂CN₂)] (3) in 56% yield (Scheme 3). The attainment of 3 differs from the formation of ylide complex in the reaction of [(κ³-N, N, P-^dfpN₂P^Ph)Fe(THF)₂] with (p-tolyl)₂CN₂, which demonstrates the effect of the ligand subsituents on the stability of the iron diazoalkane complexes. The attempts to access alkylidene iron species by thermolysis of 3 in benzene gave an intractable mixture.

  Scheme 3

  

  Scheme 3.  Reaction of 1 with (p-tolyl)₂CN₂

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  Complex 3 has been characterized by ¹H NMR, ¹³C NMR, and ³¹P NMR spectroscopies, ⁵⁷Fe Mössbauer spectroscopy, infrared spectroscopy, single-crystal X-ray diffraction, as well as elemental analysis. The molecular structure of 3 established by X-ray diffraction study revealed a side-on coordination mode of the diazoalkane ligand that has the Fe—N2 and Fe—N1 distances of 0.17524(11) and 0.19601(11) nm, respectively (Figure 2). The Fe1—N2 distance is obviously shorter than its congener in the side-on diazoalkane complexes [(PDI)Fe(η²-N, N-N₂CPh₂)]^[22] (0.1903 nm) and [L^Bu-tFe(η²-N, N-N₂CPh₂)-K(OEt₂)]₂^[20] [0.1839(2) nm], and is more close to the Fe—N distances of the end-on diazoalkane complexes [(IPr)Fe(NAr^Trip)(η¹-N₂C(tolyl-p)₂)]^[26] [0.1727(2) nm], [(tpe)Fe(η¹-N₂CPh₂)][Li(THF)₄]^[19] [0.1781(9) nm], and [L^Bu-tFe(η¹-N₂CPh₂)][0.1752(1) nm], reaching the long end of the Fe—N distances of terminal iron imido complexes.^[26] The N—N and N—C bonds of the side-on diazoalkane ligand with the distances of 0.13292(16) and 0.13054(17) nm, respectively, are comparable to those of the middle transition-metal complexes that feature dianionic diazo ligands, e.g. [Mo(OBu^t)₄(NNPh₂)]^[27] and [Cr(OSi-Bu^t₃)₂(NNCPh₂)₂].^[28] As compared to the diazo radical ligands, the N—N and N—C bonds in 3 are longer and shorter, respectively, than their counterparts in [(tpe)Fe(η¹-N₂CPh₂)][Li(THF)₄]^[19] and [L^Bu-tCo(η¹-N₂CPh₂)].^[20] The characteristic bond distances of the diazo ligand in 3, in addition with the short Fe—N (amido) [0.18571(11) and 0.18559(11) nm] and Fe—P [0.21628(4) nm] distances seem to agree with the assignment of 3 as an iron(IV) complex having a dianionic diazo ligand [η²-N, N-(p-tolyl)₂-CN₂)]^2-.

  Figure 2

  

  Figure 2.  Molecular structure of complex 3 (hydrogen atoms were omitted)

  Selected distances (nm) and angles (°): Fe1—P1, 0.21628(4); Fe1—N3, 0.18571(11); Fe1—N4, 0.18559(11); Fe1—N1, 0.19601(11); Fe1—N2, 0.17524(11); N3—Fe1—P1, 85.38(3); N4—Fe1—P1, 85.93(4); N4— Fe1—N3, 128.91(5)

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  Theoretical studies on 3 have further corroborated the assignment of 3 as an iron(IV) complex having a dianionic diazo ligand. Single point energy calculations at the B3- LYP/TZVP/SVP level of theory based on the molecular structure of 3 established by X-ray diffraction study indicate that the complex at its S＝0 spin state is 55.76 and 128.41 kJ/mol, respectively, lower in energy than those of the S＝1 and S＝2 spin states. Localized orbital analysis on the S＝0 state revealed the presence of both σ- and π-type interactions between the iron center and the terminal nitrogen atom of the diazo ligand (Figure 3). Meanwhile, the N—N and N—C bonds of the diazo ligand are found single- and double-bond in character, respectively (Figure 4). The composition of the frontier orbitals (Figure 3) points to an electronic configuration of (d_xy)²(d_xz)²(d_x-y²²)⁰(d_z²)⁰(d_yz)⁰ for the covalent iron(IV) complex 3. Being supportive to this assignment, complex 3 is diamagnetic and its zero-field ⁵⁷Fe Mössbauer data measured at 80 K (δ＝0.02 mm/s and |ΔE_Q|＝2.98 mm/s) is in reasonable agreement with those of the calculated ones (δ＝-0.08 mm/s and ΔE_Q＝3.61 mm/s).

  Figure 3

  

  Figure 3.  Selected localized orbitals for 3 (hydrogen atoms and substituents on ligands were omitted)

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  Figure 4

  

  Figure 4.  Selected localized orbitals for the N—N and N—C bonds in 3 (hydrogen atoms and substituents on ligands were omitted)

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  2.4   Reaction of 1 with PhC(N₂)CO₂Et

  Complex 1 can also readily react with the diazo compound PhC(N₂)CO₂Et at room temperature, producing the diazo iron complex [(κ³-N, N, P-^MesN₂P^Bu-t)Fe(κ²-N, O-PhC(N₂)CO₂Et)] (4) in 60% isolated yield (Scheme 4). As isolated, complex 4 presents a brown-green crystalline solid, and is sensitive to air and water. Its solution magnetic moment (μ_eff＝3.2(1)μ_B in C₆D₆) indicates an S＝1 ground state. The zero-field ⁵⁷Fe Mössbauer spectrum of 4 measured at 80 K features one quadrupole doublet with δ＝0.25 mm/s and |ΔE_Q|＝2.55 mm/s. The Mössbauer data are apparently different from those of 1 or 3, hinting at the distinct electronic nature of their iron centers (Figure 5). The attempts to access alkylidene iron complexes by thermolysis of 4 in benzene were unsuccessful.

  Scheme 4

  

  Scheme 4.  Reaction of 1 with PhC(N₂)CO₂Et

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  Figure 5

  

  Figure 5.  ⁵⁷Fe Mössbauer spectra (80 K, zero-field) of 1 (top), 3 (middle), and 4 (bottom)

  The data (dots) and best fits (solid line) are shown. The fitting data are compiled in Table 1

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  Table 1

  

  Table 1.  Zero-Field ⁵⁷Fe Mössbauer spectroscopic data of 1~5 measured at 80 K

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  Complex	δ/(mm•s-1)	|ΔEQ|/(mm•s-1)
  1	0.75	1.19
  2	0.82	2.40
  3	0.02	2.98
  4	0.25	2.55
  5	0.70	1.24

  The solid-state structure of 4, determined by X-ray crystallography, revealed that the diazo ligand is coordinating to the iron center via its terminal nitrogen atom and carbonyl oxygen atom, forming a planar six-membered chelating ring (Figure 6). Thus, complex 4 is among the rare examples of metal complexes featuring chelating diazo-ketone/ester ligands after [RhCl(PPrⁱ₃)₂(κ²-N, O-Me₂-CHCH₂C(O)C(Ph)N₂)]^[15] and [(Bu^t₂P(NSiMe₃)₂)Cu(κ²-N, O-O(N₂)C₂(C₁₂H₈))].^[29] The distances of Fe1—N3 and Fe1—O1 bonds in 4 [0.1873(2) and 0.19696(17) nm, respectively] are typical as single bonds. The N3—N4, N4—C36, C36—C35, C35—O1 distances [0.1212(3), 0.1353(3), 0.1414 and 0.1248 nm, respectively] deviate from those of the single and double bonds, indicative of electron-delo-calization within the chelating ring.

  Figure 6

  

  Figure 6.  Molecular structure of 4 (hydrogen atoms were omitted)

  Selected distances (nm) and angles (°): Fe1—P1, 0.22431(8); Fe1—O1, 0.19696(17); Fe1—N1, 0.19258(19); Fe1—N2, 0.19382(19); Fe1—N3, 0.1874(2); O1—Fe1—P1, 169.88(5); N1—Fe1—P1, 83.23(6); N1— Fe1—O1, 93.65(7); N1—Fe1—N2, 138.58(9); N2—Fe1—P1, 83.55(6); N2—Fe1—O1, 92.61(8)

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  Density functional theory (DFT) calculations have been used to further probe the electronic structure of 4. The calculation study at the B3LYP/TZVP/SVP level of theory indicates that the complex has an S＝1 ground spin-state that lies lower in energy than the S＝0 and S＝2 spin states by 70.27 and 57.23 kJ/mol, respectively. The calculated S＝1 ground spin-state is consistent with that referring from the solution magnetic susceptibility of 4. At the ground spin state, maximal alignment of the α- and β-spins of the molecular orbital diagram indicated significant spin polarization, where one electron pair has an orbital overlap of 76% between an iron 3d orbital and the π-orbital of the N—N moiety of the diazo ligand (Figure 7), indicating an electronic structure of an intermediate spin iron(III) (S_Fe＝3/2) antiferromagnetically coupled with a ligand-based radical (S_diazo＝1/2) for 4. In line with this assignment, Fe—N [0.19258(19) and 0.19382(19) nm] and Fe—P [0.22431(8) nm] distances between the amido-phosphine-amido ligand and the iron center are found to be locating between those of 1 and 3 that have high-spin iron(II) and low-spin iron(IV) centers, respectively.

  Figure 7

  

  Figure 7.  Frontier molecular orbital diagram for 4 at an S＝1 spin-state

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  2.5   Reactions of 2 with diazo compounds

  The amido-posphine-amido ligand in [(κ³-N, N, P-^dcpN₂-P^Bu-t)Fe(OEt₂)] (2) has 2, 6-dichlorophenyl as its N-substituents that should have different steric and electronic character when compared to the mesityl groups in 1. Examining the reaction of 2 with (p-tolyl)₂CN₂ revealed the capability of the iron complex in catalzying the conversion of (p-tolyl)₂CN₂ to (p-tolyl)₂C＝NN＝C(tolyl-p)₂, wherein the reaction in an 1:1 ratio produced the azine along with the majority of 2 remained unreactive according to ¹H NMR analysis. With a catalyst loading of 5 mol% 2, the reaction at room temperature in C₆D₆ for 1 h produced (p-tolyl)₂C＝NN＝C(tolyl-p)₂ in 73% NMR yield (Scheme 5). The result hints that the iron(IV) alkylidene intermediate (κ³-N, N, P-^dcpN₂P^Bu-t)Fe(C(tolyl-p)₂) might be formed during the reaction course. This intermediate, however, might be highly active and susceptible to the attack by (p-tolyl)₂CN₂, leading to the formation of the azine (p-tolyl)₂C＝NN＝C(tolyl-p)₂.

  Scheme 5

  

  Scheme 5.  Conversion of (p-tolyl)₂CN₂ to its azine catalyzed by 2

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  The other diazo compound was examined in the reactions with 2 is DmpC(N₂)H (Dmp＝2, 6-dimesityphenyl) that bears a sterically hindered Dmp group. No reaction took place when 2 was treated with DmpC(N₂)H in toluene at room temperature. Elevating temperature to 50 ℃ could lead to the occurence of the reaction. Further workup on the reaction mxiture led to the isolation of the diamido-phos- phine ylide iron(II) complex [(κ³-N, N, C-^dcpN₂-P^Bu-tC^{H, Dmp})-Fe] (5) in 70% yield (Scheme 6). Complex 5 has been fully characterized by various spectrsocopic methods, and its moelcular structure was confirmed by single-crystal X-ray diffraction study (Figure 8). The formation of 5 implies the readiness of the mono-substitued alkylidenemetal species (κ³-N, N, P-^dcpN₂P^Bu-t)Fe[CH(Dmp)] to undergo migratory insertion reaction in spite of the presence of the bulky substituents on the phosphine and alkylidene moieties.

  Scheme 6

  

  Scheme 6.  Reaction of 2 with DmpC(N₂)H

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  Figure 8

  

  Figure 8.  Molecular structure of complex 5 (hydrogen atoms were omitted)

  Selected distances (nm) and angles (°): Fe1—Cl1, 0.26554(6); Fe1—P1, 0.28358(6); Fe(1)—N(1), 0.20286(17); Fe1—N2, 0.19863(17); Fe1—C1 0.2146(2); P(1)—C(1), 0.1786(2); Cl1—Fe1—P1, 157.063(19); N1— Fe1—Cl1, 75.25(5); N1—Fe1—P1, 82.53(5); N1—Fe1—C1, 88.10(7); N2—Fe1—Cl1, 115.76(5) N2—Fe1—P1, 77.93(5); N2—Fe1—N1, 112.567; N2—Fe1—C1, 111.30(7); C1—Fe1—Cl1, 132.93(5); C1— Fe1—P1, 39.01(5)

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  3.   Conclusions

  Two new high-spin iron(II) complexes supported by tripodal amido-phosphine-amido ligands [(κ³-N, N, P-^MesN₂P^Bu-t)Fe(OEt₂)] (1) and [(κ³-N, N, P-^dcpN₂P^Bu-t)-Fe(OEt₂)] (2) were synthesized, and their reactions with diazoalkane compounds were examined. Complex 1 can react with the diazo compounds (p-tolyl)₂CN₂ and PhC-(N₂)CO₂Et to form the iron diazo complexes [(κ³-N, N, P-^MesN₂P^Bu-t)Fe(η²-N, N-(p-tolyl)₂CN₂)] (3) and [(κ³-N, N, P-^MesN₂P^Bu-t)Fe(κ²-N, O-PhC(N₂)CO₂Et)] (4). Spectroscopic characterization in combination with DFT studies suggest that 3 is a low-spin iron(IV) complex featuring a dianionic diazo ligand [η²-(p-tolyl)₂CN₂)]^2-, whereas 4 can be viewed as an intermediate iron(III) complex bearing an antiferromagnetically coupled diazo radical anion [κ²-N, O-PhC(N₂)CO₂Et)]^•1-. Reactivity studies of 2 revealed its ability in catalyzing the conversion of (p-tolyl)₂CN₂ to its azine, and its reaction with DmpC(N₂)H was found to produce the high-spin iron(II) ylide complex [(κ³-N, N, C-^dcpN₂P^Bu-tC^{H, Dmp})Fe] (5). These results demonstrated that the diversified reactivity of iron(II) species supported by tripodal amido-phosphine-amido scaffolds toward diazoalkenes.

  4.   Experimental section

  4.1   General procedures

  All experiments were performed either under an atmosphere of dry dinitrogen with the rigid exclusion of air and moisture using standard Schlenk techniques or in a glovebox. Organic solvents were dried with a solvent purification system (Innovative Technology) and bubbled with dry N₂ gas prior to use. [Fe(N(TMS)₂)₂]₂, ^[30] (p-tolyl)₂CN₂, ^[31] DmpC(N₂)H^[32] were synthesized according to literature procedures. All other chemicals were purchased from either Strem or J & K Chemical Co. and used as received unless otherwise noted. ¹H NMR, ¹³C NMR and ³¹P NMR spectra were recorded on an Agilent 400 MHz spectrometers. Chemical shifts were reported with references to the residue of the deuterated solvents for proton and carbon chemical shifts, and to external 85% phosphoric acid solution for phosphorous chemical shifts. For the ¹H NMR data, the widths of peaks at their half heights are denoted as v_1/2. Elemental analyses were performed by the Analytical Laboratory of Shanghai Institute of Organic Chemistry (CAS). Magnetic moments were measured by the method originally described by Evans with stock and experimental solutions containing a known amount of (CH₃)₃SiOSi(CH₃)₃ standard.^[33] IR spectra were recorded with a NICOLET AVATAR 330 FT-IR spectrophotometer.

  4.2   X-ray structure determination

  Crystals were coated with Paratone-N oil and mounted on a Bruker APEX CCD-based diffractometer equipped with an Oxford low-temperature apparatus. Cell parameters were retrieved with SMART software and refined using SAINT software on all reflections. Data integration was performed with SAINT, which corrected for Lorentz polarization and decay. Absorption corrections were applied using SADABS.^[34] Space groups were assigned unambiguously by analysis of symmetry and systematic absences determined by XPREP. All structures were solved and refined using SHELXTL SADABS.^[34]Metal and first coordination sphere atoms were located from direct-methods E-maps. Non-hydrogen atoms were found in alternating difference Fourier synthesis and least-squares refinement cycles, and were refined anisotropically during final cycles. The crystal data, and summary of data collection and refinement for the complexes were summarized in Tables S1. CCDC 2006056~2006059 contain the supplementary crystallographic data for complexes 2~4, and 5•Et₂O. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif.

  4.3   ⁵⁷Fe Mössbauer spectroscopy

  All solid samples for ⁵⁷Fe Mössbauer spectroscopy were run on non-enriched samples of the as-isolated complexes. Each sample was loaded into a Delrin Mössbauer sample cup for measurements and loaded under liquid nitrogen. Low temperature ⁵⁷Fe Mössbauer measurements were performed using a SeeCo. MS4 Mössbauer spectrometer integrated with a Janis SVT-400T He/N₂ cryostat for measurements at 80 K. Isomer shifts were determined relative to α-Fe at 298 K. All Mössbauer spectra were fitted using the program WMoss (SeeCo).

  4.4   Preparation of H₂(^MesN₂P^Bu-t)

  To an Et₂O (15 mL) solution of (2, 4, 6-Me₃C₆H₂)(2-Br- C₆H₄)NH (1.45 g, 5 mmol) was added ⁿBuLi (2.5 mol•L^-1 in n-hexane, 4 mL, 10 mmol) slowly at -78 ℃. The resulting mixture was allowed to warm to room temperature and further stirred for 5 h. To this solution, an Et₂O solution (10 mL) of ^tBuPCl₂ (0.40 g, 2.5 mmol) was added at -78 ℃. The resulting mixture was allowed to warm to room temperature and further stirred for 48 h. The mixture was then quenched with a saturated aqueous solution of NaHCO₃, and extracted with CH₂Cl₂ (50 mL×3). The combined organic phases were washed with a saturated NaHCO₃ aqueous solution, brine, and dried over Na₂SO₄. After filtration and removal of volatiles, the crude product was purifiedby flash column chromatography [silica gel, n-hexanes/CH₂Cl₂ (V:V＝10:1) as elute] to give H₂(^MesN₂P^Bu-t) as a white solid (0.64 g, 50% yield). m.p. 165~168 ℃; ¹H NMR (400 MHz, CDCl₃, 293 K) δ: 7.63~7.55 (m, 2H), 7.10 (dd, J＝11.3, 4.0 Hz, 2H), 6.95 (s, 2H), 6.83 (s, 2H), 6.77 (t, J＝7.4 Hz, 2H), 6.27 (d, J＝7.6 Hz, 2H), 6.19~6.16 (m, 2H), 2.31 (s, 6H), 2.21 (s, 6H), 1.64 (s, 6H), 1.53 (d, J＝13.7 Hz, 9H); ¹³C NMR (101 MHz, CDCl₃, 293 K) δ: 149.87 (d, J_C—P＝17.8 Hz), 136.23, 135.81 (d, J_C—P＝2.0 Hz), 135.53, 135.21, 134.21, 130.04, 129.09, 118.39 (d, J_C—P＝11.3 Hz), 117.58, 111.80 (d, J_C—P＝2.8 Hz), 31.69 (d, J_C—P＝9.1 Hz), 28.80 (d, J_C—P＝13.8 Hz), 21.04, 18.40, 17.50; ³¹P NMR (162 MHz, CDCl₃, 293 K) δ: -28.42; IR (film) v: 3360 (w), 2944 (w), 2917 (w), 2859 (w), 1585 (m), 1487 (s), 1438 (s), 1275 (m), 1158 (w), 1033 (w), 1011 (w), 854 (m), 809 (m), 746 (s), 683 (w), 478 (m) cm^-1. HRMS (ESI) calcd for C₃₄H₄₂N₂P [M+H]⁺: 509.3080; found 509.3081

  4.5   Preparation of H₂(^dcpN₂P^Bu-t)

  To an Et₂O (40 mL) solution of (2, 6-Cl₂C₆H₃)(2-Br-C₆H₄)NH (3.17 g, 10.0 mmol) was added ⁿBuLi (2.5 mol/L in n-hexane, 8 mL, 20 mmol) slowly at -78 ℃. The resulting mixture was allowed to warm to room temperature and further stirred for 5 h. To this solution, an Et₂O solution (10 mL) of ^tBuPCl₂ (0.80 g, 5 mmol) was added at -78 ℃. The resulting mixture was allowed to warm to room temperature and further stirred for 48 h. The mixture was then quenched with a saturated aqueous solution of NaHCO₃, and extracted with CH₂Cl₂ (50 mL×3). The combined organic phases were washed with a saturated NaHCO₃ aqueous solution, brine, and dried over Na₂SO₄. After filtration and removal of volatiles, the crude product was purifiedby flash column chromatography [silica gel, n-hexanes/CH₂Cl₂ (V:V＝10:1) as elute] to give H₂(^dcpN₂P^Bu-t) as a white solid (1.50 g, 53% yield). m.p. 220~222 ℃; ¹H NMR (400 MHz, CDCl₃, 293 K) δ: 7.59~7.49 (m, 2H), 7.39~7.19 (m, 4H), 7.14 (t, J＝7.7 Hz, 2H), 7.00 (t, J＝8.1 Hz, 2H), 6.89 (t, J＝7.4 Hz, 2H), 6.64 (d, J＝5.9 Hz, 2H), 6.33 (dd, J＝7.6, 5.5 Hz, 2H), 1.46 (d, J＝13.6 Hz, 9H); ¹³C NMR (101 MHz, CDCl₃, 293K) δ: 147.18 (d, J_C—P＝18.4 Hz), 136.98, 136.95, 134.22, 132.31~131.94 (m), 129.55, 128.70 (d, J_C—P＝30.6 Hz), 125.55, 121.65 (d, J_C—P＝13.3 Hz) 120.22, 114.60, 31.77 (d, J_C—P＝10.3 Hz), 28.71 (d, J_C—P＝13.8 Hz); ³¹P NMR (162 MHz, CDCl₃, 293K) δ: -24.43; IR (film) v: 3330 (w), 3298 (w), 2957 (w), 2863 (w), 1569 (m), 1493 (m), 1439 (s), 1411 (m), 1296 (m), 1277 (m), 1192 (w), 1156 (w), 1095 (w), 838 (s), 773 (s), 504 (w) cm^-1. HRMS (ESI) calcd for C₂₈H₂₆N₂PCl₄ [M+H]⁺: 561.0582; found 561.0571.

  4.6   Preparation of [(κ³-N, N, P-^MesN₂P^Bu-t)Fe(OEt₂)] (1)

  To a toluene (20 mL) solution of H₂(^MesN₂P^Bu-t) (125 mg, 0.25 mmol) was added [Fe(N(TMS)₂)₂]₂ (111 mg, 0.15 mmol). The resulting mixture was stirred at 50 ℃ for 2 h to afford a red solution. After removal of the solvent under vacuum, the red residue was washed by n-hexane (10 mL) to produce 1 as an orange solid (77 mg, 49%). m.p. > 163 ℃ (dec.); ¹H NMR (400MHz, C₆D₆, 293 K) δ: 64.08 (very br), 59.64 (v_1/2＝49 Hz), 47.72 (v_1/2＝87 Hz), 39.46 (v_1/2＝75 Hz), 37.57 (v_1/2＝93 Hz), 29.53 (v_1/2＝246 Hz), 23.22 (v_1/2＝51 Hz), 10.12 (very br), 6.21 (very br), 3.93 (v_1/2＝107 Hz), -37.23 (v_1/2＝86 Hz); IR (film) v: 2966 (s), 2916 (s), 2864 (m), 1586 (s), 1452 (s), 1431 (s), 1312 (m), 1266 (m), 1201 (m), 1039 (m), 876 (3), 856 (m), 748 (s) cm^-1. Magnetic susceptibility (C₆D₆, 293 K): μ_eff＝4.7(1)μ_B. Anal. calcd for C₃₈H₄₉FeN₂OP: C 71.69, H 7.76, N 4.40; found C 71.55, H 7.52, N 4.38. ⁵⁷Fe Mössbauer spectrum (Zero-field, 80 K) δ: 0.75 mm/s, |ΔE_Q|＝1.19 mm/s.

  4.7   Preparation of [(κ³-N, N, P-^dcpN₂P^Bu-t)Fe(OEt₂)] (2)

  To a tetrahydrofuran (THF) (20 mL) solution of H₂(^dcpN₂- P^Bu-t) (105 mg, 0.19 mmol) was added [Fe(N(TMS)₂)₂]₂ (106 mg, 0.14 mmol). The resulting mixture was stirred at room temperature for 2 h to afford a orange solution. After removal of the solvent under vacuum, the orange residue was washed by n-hexane (10 mL) to produce 2 as an orange solid (100 mg, 77%). Single-crystals of 2 suitable for X-ray diffraction study were obtained by standing its saturated Et₂O solution at room temperature via slow evaporation of Et₂O overnight. m.p. > 201 ℃ (dec.); ¹H NMR (400 MHz, C₆D₆, 293 K) δ: 41.55 (v_1/2＝56 Hz), 36.74 (v_1/2＝63 Hz), 33.97 (v_1/2＝95 Hz), 29.80 (v_1/2＝179 Hz), 16.53 (v_1/2＝41 Hz), 5.17 (very br), -8.68 (v_1/2＝128 Hz), -27.67 (v_1/2＝45 Hz), -40.58 (very br), -42.64 (v_1/2＝49 Hz); IR (film) v: 1583 (s), 1453 (s), 1450 (s), 1316 (m), 1071 (m), 1032 (m), 863 (m), 780 (s), 766 (s), 748 (m) cm^-1. Magnetic susceptibility (C₆D₆, 293 K): μ_eff＝5.0(1)μ_B. Anal. calcd for C₃₂H₃₃FeN₂OCl₄P: C 55.68, H 4.82, N 4.06; found C 55.57, H 4.80, N 4.10. ⁵⁷Fe Mössbauer spectrum (Zero-field, 80 K) δ: 0.82 mm/s, |ΔE_Q|＝2.40 mm/s.

  4.8   Preparation of [(κ³-N, N, P-^MesN₂P^Bu-t)Fe(η²-N, N-(p-tolyl)₂CN₂)] (3)

  To a red solution of 1 (217 mg, 0.34 mmol) in toluene (15 mL) was added (p-tolyl)₂CN₂ (76 mg, 0.34 mmol) at room temperature for 2 h. The color of the solution changed from red to brown gradually. After removal of the solvent under vacuum, the brown residue was washed by n-pentane (10 mL) to produce 3 as a brown solid (149 mg, 56% yield). Single-crystals of 3 suitable for X-ray diffraction study were obtained by standing its saturated n-pentane solution at room temperature via slow evaporation of n-pentane overnight. m.p. > 167 ℃ (dec.); ¹H NMR (400 MHz, C₆D₆, 293 K) δ: 7.77 (d, J＝7.8 Hz, 2H), 7.31 (t, J＝7.3 Hz, 2H), 7.02 (d, J＝7.8 Hz, 2H), 6.94 (t, J＝7.4 Hz, 2H), 6.61 (t, J＝7.0 Hz, 4H), 6.43 (s, 4H), 6.42~6.34 (m, 2H), 5.80 (d, J＝7.9 Hz, 2H), 2.41 (s, 6H), 2.07 (d, J＝20.5 Hz, 6H), 1.94 (s, 6H), 1.62 (d, J＝15.4 Hz, 9H), 1.54 (s, 6H); ¹³C NMR (101 MHz, C₆D₆, 293 K) δ: 170.99 (d, J_C—P＝24.5 Hz), 146.12, 138.33, 137.34, 136.18, 134.67, 134.16, 133.74, 133.54, 131.51, 130.84, 129.25, 129.20, 129.04, 128.86, 118.68, 115.53, 115.12, 114.61, 114.50, 33.35 (d, J_C—P＝29.9 Hz), 27.80, 21.34 (d, J_C—P＝7.2 Hz), 21.04, 20.07, 18.40; ³¹P NMR (162 MHz, C₆D₆, 293 K) δ: 108.22; IR (film) v: 2967 (w), 2918 (m), 2867 (m), 2035 (s), 1593 (m), 1512 (s), 1451 (s), 1432 (s), 1316 (m), 1272 (m), 1064 (m), 1041 (m), 858 (w), 813 (m), 749 (m), 623 (w), 566 (w) cm^-1. Anal. calcd for C₄₉H₅₃FeN₄P: C 74.99, H 6.81, N 7.14; found C 75.20, H 6.75, N 7.40. ⁵⁷Fe Mössbauer spectrum (Zero-field, 80 K) δ: 0.02 mm/s, |ΔE_Q|＝2.98 mm/s.

  4.9   Preparation of [(κ³-N, N, P-^MesN₂P^Bu-t)Fe(κ²-N, O-PhC(N₂)CO₂Et)] (4)

  To a red solution of 1 (151 mg, 0.24 mmol) in toluene (15 mL) was added PhC(N₂)CO₂Et (45 mg, 0.24 mmol) at room temperature for 2 h. The color of the solution changed from red to green gradually. After removal of the solvent under vacuum, the green residue was washed by n-pentane (10 mL) to produce 4 as a green solid (108 mg, 60% yield). Single-crystals of 4 suitable for X-ray diffraction study were obtained by standing its saturated n-pentane solution at room temperature via slow evaporation of n-pentane overnight. m.p. > 160 ℃ (dec.); ¹H NMR (400 MHz, C₆D₆, 293 K) δ: 34.16 (v_1/2＝41 Hz), 26.71 (v_1/2＝40 Hz), 24.59 (v_1/2＝24 Hz), 22.33 (v_1/2＝20 Hz), 15.61 (v_1/2＝31 Hz), -0.83 (v_1/2＝78 Hz), -31.30 (v_1/2＝45 Hz), -31.63 (v_1/2＝25 Hz), -52.10 (v_1/2＝47 Hz); IR (film) v: 2994 (w), 2963 (w), 2920 (w), 2090 (m), 1709 (m), 1577 (s), 1435 (s), 1371 (m), 1288 (s), 1276 (s), 1255 (s), 1195 (m), 876 (m), 756 (m), 741 (w) cm^-1. Magnetic susceptibility (C₆D₆, 293 K): μ_eff＝3.2(1)μ_B. Anal. calcd for C₄₄H₄₉FeN₄O₂P: C 70.21, H 6.56, N 7.44; found C 69.49, H 6.45, N 7.37. ⁵⁷Fe Mössbauer spectrum (Zero-field, 80 K) δ: 0.25 mm/s, |ΔE_Q|＝2.55 mm/s.

  4.10   Catalytic conversion of (p-tolyl)₂CN₂ mediated by 2

  To a C₆D₆ solution (0.2 mL) of 2 (5.1 mg, 0.007 mmol) in a J-Young tube was added (p-tolyl)₂CN₂(33 mg, 0.148 mmol) and C₆D₆ (0.3 mL) at room temperature. After 1 h, an internal standard 1, 3, 5-trimethoxybenzene (2.1 mg, 0.013 mmol) was then added. ¹H NMR analysis on the reaction mixture indicated the formation of the (p-tolyl)₂- C＝NN＝C(p-tolyl)₂ in 73% yield.

  4.11   Preparation of [(κ³-N, N, C-^dcpN₂P^Bu-tC^{H, Dmp})Fe] (5)

  To an orange solution of 2 (267 mg, 0.47 mmol) in toluene (15 mL) was added DmpC(N₂)H (202 mg, 0.57 mmol). The mixture was then heated at 50 ℃ for 2 h. The color of the solution changed from orange to yellow-green gradually. After removal of the solvent under vacuum, the yellow-green residue was washed by n-hexane (10 mL) and dried under vaccum, which leaved 5 as a yellow-green solid (270 mg, 70% yield). Single-crystals of 5 suitable for X-ray diffraction study were obtained by standing its saturated Et₂O solution at room temperature via slow evaporation of Et₂O overnight. m.p. > 190 ℃ (dec.); ¹H NMR (400 MHz, C₆D₆, 293 K) δ: ¹H NMR (400 MHz, C₆D₆, 293 K) δ: 61.44 (v_1/2＝42 Hz), 44.44 (v_1/2＝31 Hz), 39.97 (v_1/2＝35 Hz), 36.05 (v_1/2＝32 Hz), 31.52 (v_1/2＝49 Hz), 30.36 (v_1/2＝55 Hz), 28.31 (v_1/2＝49 Hz), 23.74 (v_1/2＝29 Hz), 19.32 (v_1/2＝29 Hz), 17.62 (very br), 16.82 (v_1/2＝36 Hz), 16.19 (v_1/2＝22 Hz), 15.45 (v_1/2＝54 Hz), 13.99 (v_1/2＝14 Hz), 12.73 (v_1/2＝21 Hz), 9.43 (v_1/2＝28 Hz), -7.61 (v_1/2＝203 Hz), -9.10 (very br), -15.32 (v_1/2＝28 Hz), -22.49 (v_1/2＝28 Hz), -26.67 (v_1/2＝28 Hz), -36.60 (very br), -36.68 (very br), -37.36 (v_1/2＝32 Hz), -74.56 (very br); IR (film) v: 2963 (m), 2916 (m), 2864 (m), 1587 (m), 1553 (w), 1468 (s), 1454 (w), 1443 (s), 1299 (m), 1293 (m), 1093 (w), 1067 (w), 867 (w), 847 (m), 773 (m), 743 (m) cm^-1. Magnetic susceptibility (C₆D₆, 293 K): μ_eff＝4.9(1)μ_B. Anal. calcd for C₅₂H₄₉Cl₄FeN₂P: C 67.53, H 5.24, N 2.97; found C 67.37, H 5.34, N 3.01. ⁵⁷Fe Mössbauer spectrum (Zero-field, 80 K) δ: 0.70 mm/s, |ΔE_Q|＝1.24 mm/s.

  4.12   Computational details

  To have a better understanding of the electronic structures of 3 and 4, density functional theory (DFT)^[35-36] study has been performed with the ORCA 4.1.0 program^[37] using the B3LYP methods.^[38-39] The SVP^[40-41] basis set was used for the C, N, and H atoms, and the TZVP^[42] basis set was used for the other atoms. The RIJCOSX^[43] approximation with matching auxiliary basis sets^[40-41] was employed to accelerate the calculations. The calculation utilizes the atom-pairwise dispersion correction with the Becke-John- son damping scheme (D3BJ).^[44-45] The atomic coordinates of 3 and 4 used for calculation were obtained from X-ray diffraction studies, and only the positions of hydrogen atoms were optimized. The selected localized orbitals for 3 are listed in Figures 3 and 4. The frontier molecular orbital diagram for 4 is listed in Figure 6.

  Supporting Information  NMR, IR, Mössbauer spectra for the new complexes, Table for crystal data (PDF). The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn.

  
  
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• 

  Figure Chart 1  Examples of isolable iron diazoalkane complexes

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  Scheme 1  Reaction of [(κ³-N, N, P-^dfpN₂P^Ph)Fe(THF)₂] with (p-tolyl)₂CN₂

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  Scheme 2  Synthetic route to the (amido-phosphine-amido)iron(II) complexes 1 and 2

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  Figure 1  Molecular structure of 2 (hydrogen atoms were omitted)

  Selected distances (nm) and angles (°): Fe1—N1, 0.19877(14); Fe1—N2, 0.19926(14); Fe1—P1, 0.24116(5); Fe1—Cl1, 0.2845; Fe1—O1, 0.20626(13); N1—Fe1—N2, 123.31(6); N1—Fe1—P1, 80.99(4); N1—Fe1—O1, 113.78(6); N2—Fe1—P1, 84.25(4); N2—Fe1—O1, 121.62(6); P1— Fe1—O1, 117.17(4); P1—Fe1—Cl1, 153.00

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  Scheme 3  Reaction of 1 with (p-tolyl)₂CN₂

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  Figure 2  Molecular structure of complex 3 (hydrogen atoms were omitted)

  Selected distances (nm) and angles (°): Fe1—P1, 0.21628(4); Fe1—N3, 0.18571(11); Fe1—N4, 0.18559(11); Fe1—N1, 0.19601(11); Fe1—N2, 0.17524(11); N3—Fe1—P1, 85.38(3); N4—Fe1—P1, 85.93(4); N4— Fe1—N3, 128.91(5)

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  Figure 3  Selected localized orbitals for 3 (hydrogen atoms and substituents on ligands were omitted)

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  Figure 4  Selected localized orbitals for the N—N and N—C bonds in 3 (hydrogen atoms and substituents on ligands were omitted)

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  Scheme 4  Reaction of 1 with PhC(N₂)CO₂Et

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  Figure 5  ⁵⁷Fe Mössbauer spectra (80 K, zero-field) of 1 (top), 3 (middle), and 4 (bottom)

  The data (dots) and best fits (solid line) are shown. The fitting data are compiled in Table 1

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  Figure 6  Molecular structure of 4 (hydrogen atoms were omitted)

  Selected distances (nm) and angles (°): Fe1—P1, 0.22431(8); Fe1—O1, 0.19696(17); Fe1—N1, 0.19258(19); Fe1—N2, 0.19382(19); Fe1—N3, 0.1874(2); O1—Fe1—P1, 169.88(5); N1—Fe1—P1, 83.23(6); N1— Fe1—O1, 93.65(7); N1—Fe1—N2, 138.58(9); N2—Fe1—P1, 83.55(6); N2—Fe1—O1, 92.61(8)

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  Figure 7  Frontier molecular orbital diagram for 4 at an S＝1 spin-state

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  Scheme 5  Conversion of (p-tolyl)₂CN₂ to its azine catalyzed by 2

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  Scheme 6  Reaction of 2 with DmpC(N₂)H

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  Figure 8  Molecular structure of complex 5 (hydrogen atoms were omitted)

  Selected distances (nm) and angles (°): Fe1—Cl1, 0.26554(6); Fe1—P1, 0.28358(6); Fe(1)—N(1), 0.20286(17); Fe1—N2, 0.19863(17); Fe1—C1 0.2146(2); P(1)—C(1), 0.1786(2); Cl1—Fe1—P1, 157.063(19); N1— Fe1—Cl1, 75.25(5); N1—Fe1—P1, 82.53(5); N1—Fe1—C1, 88.10(7); N2—Fe1—Cl1, 115.76(5) N2—Fe1—P1, 77.93(5); N2—Fe1—N1, 112.567; N2—Fe1—C1, 111.30(7); C1—Fe1—Cl1, 132.93(5); C1— Fe1—P1, 39.01(5)

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  Table 1.  Zero-Field ⁵⁷Fe Mössbauer spectroscopic data of 1~5 measured at 80 K

  Complex	δ/(mm•s-1)	|ΔEQ|/(mm•s-1)
  1	0.75	1.19
  2	0.82	2.40
  3	0.02	2.98
  4	0.25	2.55
  5	0.70	1.24

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