Metalloradical Cations and Dications Based on Divinyldiphosphene and Divinyldiarsene Ligands

Abstract

Metalloradicals are key species in synthesis, catalysis, and bioinorganic chemistry. Herein, two iron radical cation complexes (3‐E)GaCl₄ [(3‐E)^.+ = [{(IPr)C(Ph)E}₂Fe(CO)₃]^.+, E = P or As; IPr = C{(NDipp)CH}₂, Dipp = 2,6‐iPr₂C₆H₃] are reported as crystalline solids. Treatment of the divinyldipnictenes {(IPr)C(Ph)E}₂ (1‐E) with Fe₂(CO)₉ affords [{(IPr)C(Ph)E}₂Fe(CO)₃] (2‐E), in which 1‐E binds to the Fe atom in an allylic (η³‐EEC_vinyl) fashion and functions as a 4e donor ligand. Complexes 2‐E undergo 1e oxidation with GaCl₃ to yield (3‐E)GaCl₄. Spin density analysis revealed that the unpaired electron in (3‐E)^.+ is mainly located on the Fe (52–64 %) and vinylic C (30–36 %) atoms. Further 1e oxidation of (3‐E)GaCl₄ leads to unprecedented η³‐EEC_vinyl to η³‐EC_vinylC_Ph coordination shuttling to form the dications (4‐E)(GaCl₄)₂.

Dipnictene‐based metalloradicals: The metalloradicals (3‐E)^.+ (E=P or As) are accessible as crystalline violet solids by 1e oxidation of divinyldipnictene Fe⁰ complexes 2E. Further 1e oxidation of (3‐E)^.+ leads to unprecedented η³‐EEC_vinyl to η³‐EC_vinylC_Ph coordination shuttling resulting in the dications (4‐E)²⁺.

Introduction

The isolation and exploration of stable radicals is an area of high current interest because of their intriguing electronic structures and properties; ^[1] hence, they offer promising perspectives in synthesis, catalysis, ^[2] and materials science. ^[3] Metalloradicals based on earth‐abundant 3d metals, ^[4] which preferably undergo single‐electron transfer reactions and frequently involve open‐shell intermediates, ^[5] are of particular interest in organometallic chemistry ^[6] and sustainable synthesis. ^[7] The use of geometrically constrained alkene‐containing ligands, bulky porphyrins in particular, ^[8] is a remarkably successful approach to stabilizing mononuclear metalloradicals. ^[9]

Diphosphenes II, ^[10] the phosphorus analogues of ubiquitous alkenes I (Figure 1), are an interesting class of functional molecules as they, in addition to the P=P bond, feature a lone pair of electrons on the phosphorus atom. ^[11] Therefore, II may, in principle, serve as σ and π donors. However, the HOMO of II is invariably the lone pair at the phosphorus atom, while the P=P π‐bonding orbital is very low lying, and thus usually inaccessible for chemical bonding.[ ¹⁰ , ¹² ] Unlike alkenes I, diphosphenes II are highly reactive and their isolation requires the use of bulky substituents at the phosphorus atoms. ^[13] Over the years, a variety of metal complexes derived from stable diphosphenes and heavier congeners have been reported.[ ¹³ , ¹⁴ ] Interestingly, there is only one example of a stable diphosphene metalloradical cation (III) known to date, while analogous heavier pnictogen derivatives have remained virtually unknown. ^[15] The scarcity of stable diphosphene radical cations and anions is most likely because of the large HOMO–LUMO energy gap.[ ¹⁰ , ¹² , ¹⁶ ] The lowering of the HOMO–LUMO energy gap by π conjugation is fundamental to organic chemistry and materials science. ^[17] Recently, we reported stable dipnictenes IV ^[18] as well as pnictinidene complexes ^[19] containing N‐heterocyclic vinyl substituents derived from classical NHCs. The HOMO–LUMO energy gap of 1‐E (E=P: 4.15 eV; E=As: 3.86 eV) ^[18] is the narrowest among dipnictenes reported thus far. ^[12] Remarkably, unlike classical dipnictenes II, the HOMO of IV is mainly the E=E π bond, which is very high lying and enables the isolation of stable radical cations ^[20] (IV)^.+ by 1e oxidation of IV. The electronic features of IV further prompted us to explore their suitability as ligands in transition‐metal chemistry and to probe the synthetic viability of stable metalloradicals.

Figure 1.

Schematic representation of I–IV.

Herein, we report the synthesis of dipnictene complexes [(1‐E)Fe(CO)₃] (2‐E) and their sequential 1e oxidation to yield the corresponding stable radical cations [(1‐E)Fe(CO)₃]^.+ and dications [(1‐E)Fe(CO)₃]²⁺ (Schemes 1 and 2).

Scheme 1.

Synthesis of Fe⁰ complexes 2‐P and 2‐As.

Scheme 2.

Synthesis of (3‐E)GaCl₄ and (4‐E)(GaCl₄)₂ (E=P or As).

Results and Discussion

The Fe⁰ complexes [(1‐E)Fe(CO)₃] (2‐E) were prepared by UV irradiation of a THF solution of the respective divinyldipnictene {(IPr)C(Ph)E}₂ (1‐E) (E=P or As) and Fe₂(CO)₉ (Scheme 1). Complexes 2‐E are brown crystalline solids, soluble in common organic solvents (Et₂O, benzene, toluene, and THF), and are stable under an inert gas atmosphere. The ¹H NMR spectra of 2‐E exhibit well‐resolved resonances for the (IPr)CPh moieties, which are fully consistent with the ¹³C{¹H} NMR spectra (see the Supporting Information). The ³¹P{¹H} NMR spectrum of 2‐P shows two doublets at −9.2 and −26.6 ppm (¹ J _PP=363 Hz), indicating the presence of distinct coordination environments around the P atoms.

The solid‐state molecular structures of 2‐E determined by X‐ray diffraction studies (Figure 2) correlate well with the solution NMR data. The structures of 2‐E reveal a trans‐bent geometry along the E=E bond, which binds to the Fe atom in a side‐on fashion. The Fe atom in 2‐E is at the center of a distorted trigonal‐bipyramidal geometry and features three terminal CO ligands. One of the vinylic carbon atoms of the (IPr)C(Ph) moieties binds to the Fe atom, whereas the second remains free. Thus, like anionic allylic ligands, ^[21] the neutral 1‐E molecule serves as a 4e donor in 2‐E (see below). The E−E bond lengths of 2‐P (2.145(6) Å) and 2‐As (2.354(6) Å) are longer than those of 1‐P (2.062(1) Å) ^[18b] and 1‐As (2.296(3) Å), ^[18a] respectively (Table 1). They are, however, still shorter than the calculated E−E single‐bond lengths (2.20 Å, E=P; 2.46 Å, E=As). ^[22] The η³‐EEC_vinyl coordination of 1‐E results in deplanarization of the C₂E₂C₂ scaffold in 2‐E, which has a dihedral angle of 8.63 (2‐P) or 13.91° (2‐As). Because of the transfer of π‐electron density towards the Fe atom of the Lewis acidic Fe(CO)₃ unit, the C1−C2 bond lengths of 2‐P (1.496(5) Å) and 2‐As (1.459(1) Å) become longer and comparable to C−C single‐bond lengths. ^[22] Interestingly, the C3−C4 bond lengths of 2‐P (1.375(7) Å) and 2‐As (1.385(1) Å) remain comparable to those of 1‐P (1.387(2) Å) ^[18b] and 1‐As (1.376(2) Å), ^[18a] respectively. As expected, the C4−E2 bond lengths (1.813(8) Å in 2‐P and 1.934(1) Å in 2‐As) are shorter than the C2−E1 bond lengths (1.842(6) Å in 2‐P and 1.959(9) Å in 2‐As). These bond lengths are longer than those in 1‐P (1.785(1) Å) and 1‐As (1.919(1) Å), and thus indicate reduced π conjugation in 2‐E. The Fe−P1 (2.262(5) Å) bond in 2‐P is marginally longer than that of the η¹‐diphosphene complex [(Mes*P)₂Fe(CO)₄] (2.215(1) Å) (Mes*=2,4,6‐tBu₃C₆H₂). ^[14g] The Fe−P2 (2.475(5) Å) bond is, however, longer than the sum of the Fe and P covalent radii, indicating a weak interaction. A similar trend is also visible for the Fe−As1 (2.364(5) Å) and Fe−As2 (2.567(5) Å) bond lengths of 2‐As. The Fe−C2 bond lengths of 2‐P (2.162(6) Å) and 2‐As (2.169(1) Å) are comparable to those found for π‐allyl iron complexes (2.16 Å). ^[23]

Figure 2.

Solid‐state molecular structures of 2‐P and 2‐As (top). H and minor disordered atoms have been omitted for clarity. HOMOs of 2‐P and 2‐As (bottom) calculated at the B3LYP+D3(BJ)/def2‐TZVPP//B3LYP+D3(BJ)/def2‐SVP level of theory (isovalue 0.06). H atoms have been omitted for clarity.

Table 1.

Selected bond lengths [Å] and angles [°] of 1‐E, 2‐E, (3‐E)GaCl₄, and (4‐E)(GaCl₄)₂.

	C1−C2	C3−C4	C2−E1	C4−E2	E1−E2	Fe−C2	Fe−E1	Fe−E2/Fe−C5 [b]	C2−E1−E2	E2−Fe−C6
1‐P [a]	1.387(2)	–	1.785(1)	–	2.062(1)	–	–	–	102.6(1)	–
2‐P	1.496(5)	1.375(7)	1.842(6)	1.813(8)	2.145(6)	2.162(6)	2.262(5)	2.475(5)	98.4(2)	163.4(5)
(3‐P).+	1.470(7)	1.429(7)	1.826(5)	1.771(5)	2.197(2)	2.149(5)	2.240(2)	2.495(1)	91.4(2)	168.4(8)
(4‐P)2+	1.482(2)	1.470(3)	1.819(4)	1.700(7)	2.219(3)	2.084(2)	2.320(5)	2.190(2)	107.9(6)	–
1‐As [a]	1.376(2)	–	1.919(1)	–	2.296(3)	–	–	–	98.99(4)	–
2‐As	1.459(1)	1.385(1)	1.959(9)	1.934(1)	2.354(6)	2.169(1)	2.364(5)	2.567(5)	90.9(3)	167.4(1)
(4‐As)2+	1.476(3)	1.461(3)	1.957(3)	1.820(3)	2.454(4)	2.089(3)	2.423(5)	2.186(3)	105.9(7)	–

[a] Structures have a center of inversion. [b] The ipso‐C_Ph–Fe interaction only in the dications (4‐E)²⁺.

The optimized geometries of 2‐P and 2‐As calculated at the B3LYP+D3(BJ)/def2‐SVP level of theory (Figure S29, Tables S3 and S4 in the Supporting Information) are in good agreement with their solid‐state molecular structures. The HOMOs of 2‐P and 2‐As are π‐type orbitals (Figure 2) mainly located at the C_IPr=C_vinyl (C3=C4) bond with some contributions from the same imidazole ring and iron d orbitals. The nature of the bonding interaction between Fe(CO)₃ and 1‐E fragments to form 2‐E was analyzed by energy decomposition analysis (EDA). ^[24] According to calculations, the bonding has approximately 10 % dispersion, 42 % electrostatic, and 47 % orbital interactions. For 2‐E, EDA suggests donor–acceptor‐type interactions between Fe(CO)₃ and 1‐E fragments (Figure 3 a). The best fragmentation representation was selected by means of the orbital interaction energy term ΔE _orb, which indicates how much the orbitals relax on going from the fragments to the final state of a molecule. Thus, a small absolute value of ΔE _orb indicates that the selected electronic structure of the fragments matches with that of the real molecule. Consequently, the best electronic‐structure representation is the one that leads to the smaller absolute value of ΔE _orb (see Table S17 in the Supporting Information). The singlet electronic state of Fe(CO)₃ has a fully occupied 3d_xz orbital (e in C _3v symmetry), and two empty 3d_yz (e) and 4s (a₁) orbitals. The empty orbitals of Fe(CO₃) interact with the symmetry‐matching doubly occupied π orbitals (HOMO and HOMO−3) located at the central CE₂C moiety. The occupied orbital at Fe(CO)₃ interacts with the LUMO at the CE₂C unit.

Figure 3.

Qualitative molecular orbital diagrams showing the interactions between a) Fe(CO)₃ (C _3v symmetry) and 1‐E (E=P and As) to form 2‐E (E=P and As) and b) Fe(CO)₃ (C _3v symmetry) and (1‐E)^.+ (E=P and As) to form (3‐E)^.+ (E=P and As). For the sake of clarity, only the orbitals associated with bonding are considered.

The cyclic voltammograms (CVs) of 2‐P (E _1/2=−0.85, +0.17 V) and 2‐As (E _1/2=−0.82, +0.27 V) show two main redox events (Figure S1 in the Supporting Information), which may be assigned to the corresponding radical cations and dications, respectively. Indeed, addition of two equivalents of GaCl₃ to a red solution of 2‐P or 2‐As in CH₂Cl₂ immediately led to the formation of a violet solution. ^[25] After workup, the radical cations (3‐E)GaCl₄ were isolated as violet solids (Scheme 2). Consistent with the CVs, further oxidation of (3‐E)GaCl₄ to the dicationic species (4‐E)(GaCl₄)₂ is also viable with GaCl₃. (4‐E)(GaCl₄)₂ are red crystalline solids, which can also be prepared directly by one‐pot reactions of 2‐E with four equivalents of GaCl₃. Both (3‐E)GaCl₄ and (4‐E)(GaCl₄)₂ are stable in solution and in the solid‐state under an inert gas atmosphere. The dications (4‐E)(GaCl₄)₂ exhibit well‐resolved ¹H and ¹³C{¹H} NMR signals for the (IPr)CPh moieties. The ³¹P{¹H} NMR spectrum of (4‐P)(GaCl₄)₂ displays two doublets at +0.03 and +454.9 ppm (¹ J _PP=352 Hz), which are downfield‐shifted compared to those of 2‐P (−9.2, −26.6 ppm). The signal at +0.03 ppm is comparable with those of phosphides, ^[26] whereas the signal at +454.9 ppm is consistent with those of phosphaalkenes. ^[27]

The molecular structures of (3‐P)^.+, (4‐P)²⁺ (Figure 4), and (4‐As)²⁺ (Figure S28 in the Supporting Information) show trans‐bent geometries along the E−E bond. The η³‐EEC_vinyl coordination mode at the Fe atom remained virtually the same in 2‐P and (3‐P)^.+, except for some alternations in the bond parameters (Table 1). Interestingly, the side‐on η³‐EEC_vinyl bonding to the Fe atom in (4‐P)²⁺ and (4‐As)²⁺ does not persist, and instead the Fe atom binds to only one of the E atoms in an end‐on fashion and has a short Fe–ipso‐C_Ph contact. This leads to a new η³‐EC_vinylC_Ph coordination mode of the ligand at the Fe atom. Theoretical calculations (see below) support the preference of end‐on η³‐EC_vinylC_Ph‐coordination in (4‐P)²⁺ over the side‐on η³‐EEC_vinyl coordination, as shown by 2‐E and (3‐P)^.+. This type of redox‐induced coordination shuttling is unprecedented in dipnictene chemistry. The P1−P2 bond length of (3‐P)^.+ (2.197(2) Å) is longer than those of 2‐P (2.145(6) Å), 1‐P (2.062(1) Å), ^[18b] and (1‐P)^.+ (2.106(7) Å) ^[20a] (Table 1), but matches well with the calculated P−P single‐bond length (ca. 2.20 Å). ^[22] The C4−P2 (1.771(5) Å) and C2−P1 (1.826(5) Å) bond lengths of (3‐P)^.+ are smaller than those of 2‐P (1.813(8) and 1.842(6) Å, respectively), but longer than that of (1‐P)^.+ (1.744(2) Å). ^[20a] The C3−C4 bond length in (3‐P)^.+ (1.429(7) Å) is larger than that of 2‐P (1.375(7) Å), while the C1−C2 bond length of (3‐P)^.+ (1.470(7) Å) is marginally smaller than that of 2‐P (1.496(5) Å). The Fe1−P1 (2.240(2) Å) and Fe1−C2 (2.149(5) Å) bond lengths of (3‐P)^.+ are shorter than those of 2‐P. The Fe1−P2 (2.495(1) Å) bond length is slightly longer than that of 2‐P (2.475(5) Å). The P1−P2 bond length of (4‐P)²⁺ (2.219(3) Å) is the longest among (3‐P)^.+ (2.197(4) Å), 2‐P (2.145(6) Å), and (1‐P)²⁺ (2.192(1) Å) (Table 1). A similar trend in the As−As bond lengths can also be seen for (4‐As)²⁺ (2.454(4) Å), 2‐As (2.354(6) Å), and (1‐As)²⁺ (2.419(1) Å). In (4‐E)²⁺, the C4−P2 (1.700(7) Å) and C4−As2 (1.820(3) Å) bond lengths are smaller than those of C2−P1 (1.819(4) Å) and C2−As1 (1.957(3) Å), respectively. The former are consistent with the C=E double bond lengths of phosphaalkenes ^[28] and arsaalkenes, ^[29] while the latter are comparable to C−E (E=P or As) single‐bond lengths. ^[22] Similarly, the Fe1−As1 (2.423(5) Å) and Fe1−C2 (2.089(3) Å) bond lengths of (4‐As)²⁺ are also shorter than those of 2‐As. The C1−C2 (1.482(2) Å) bond length of (4‐P)²⁺ is slightly longer than that of (3‐P)^.+ (1.470(7) Å), but shorter with respect to that of 2‐P (1.496(5) Å).

Figure 4.

Solid‐state molecular structures of (3‐P)^.+ and (4‐P)²⁺. H atoms, solvent molecule(s), and GaCl₄ ⁻ anion(s) have been omitted for clarity (see Table 1 for selected bond angles and lengths).

The optimized geometries (Figure S29 in the Supporting Information) at the B3LYP+D3(BJ)/def2‐SVP level of theory are in good agreement with the X‐ray diffraction (Table 1) structures of (3‐P)^.+ and (4‐E)²⁺ (Tables S5–S8 in the Supporting Information). The elongation of the E−E bond length from 2‐E to (3‐E)^.+ is in line with the slight lowering of the E−E bond order from 2‐E (E=P 1.09, As 1.04) to (3‐E)^.+ (E=P 1.05, As 0.98) (Table S10 in the Supporting Information). The SOMO of (3‐E)^.+ (Figure 5 a) is the C_IPr=C_vinyl π bond with some contribution from the d orbital of the Fe(CO)₃ moiety. The LUMO of (3‐E)^.+ is the other C_IPr−C_vinyl π‐bond close to the Fe atom with a discrete contribution of the p orbital of the E atom (Figures S32 and S33 in the Supporting Information). The localized Mulliken atomic spin density (Figure 5 b) for (3‐E)^.+ reveals that the unpaired electron is mainly located at the iron atom (E=P, 0.62 e; As, 0.54 e). Notably, the spin density at the E atoms (0.02–0.08 e) is rather low compared to that at the vinylic carbon atom of (3‐E)^.+ (E=P, 0.30 e; As, 0.36 e). We also performed a Local Spin Analysis, in which the expectation value of the spin‐squared operator is decomposed into atomic (local spins) and diatomic terms. ^[30] The sum of the local spin atomic expectation values for Fe(CO)₃ (E=P, 0.379; As, 0.312) and C₂E₂ (E=P, 0.409, As, 0.467) suggest that the radical cations (3‐P)^.+ are stabilized by delocalization of the unpaired electron over the FeE₂C framework (Figure S37 in the Supporting Information). Strikingly, the oxidation of 2‐E to (3‐E)^.+and of (3‐E)^.+ to (4‐E)²⁺ weakens the interaction between (1‐E)ⁿ⁺ (n=2, 1, or 0) and Fe(CO)₃ moieties. Thus, the dissociation energies for the coordination of Fe(CO)₃ in (4‐E)²⁺ (E=P, 42.6; As, 42.4 kcal mol⁻¹) are lower than those of (3‐E)^.+ (E=P, 47.6; As, 51.5 kcal mol⁻¹), which are lower than those of 2‐E (E=P, 55.5; As, 59.8 kcal mol⁻¹) (Table S17 in the Supporting Information).

Figure 5.

a) SOMOs of (3‐P)^.+ and (3‐As)^.+ at the B3LYP+D3(BJ)/def2‐TZVPP//B3LYP+D3(BJ)/def2‐SVP level of theory (isovalue 0.06). H atoms have been omitted for clarity. b) Spin density (isovalue 0.004 a.u.) and Mulliken spin‐density plots of (3‐P)^.+ and (3‐As)^.+.

We also analyzed the nature of the bonding interactions by the EDA using different fragmentation schemes (see Figure S44 in the Supporting Information). ^[31] The coordination of Fe(CO)₃ to (1‐E)^.+ to form (3‐E)^.+ (Figure 3 b) occurs through the triplet reference state (e¹e¹a₁ ⁰) of the iron moiety. The bonding nature in (3‐E)^.+ (≈10 % dispersion, ≈47 % electrostatic, and ≈43 % orbital interactions) is similar to that of 2‐E, but the orbital interaction becomes weaker since back‐donation involves only one electron from the Fe(CO)₃ to the LUMO of (1‐E)^.+. Thus, the coordination also becomes weaker (ΔE _int=−86.4, E=P; ΔE _int=−88.6 kcal mol⁻¹, E=As).

The X‐band EPR spectrum of (3‐P)GaCl₄ (g _iso=2.0959) at 298 K exhibits a doublet of doublets due to coupling of the unpaired electron with two magnetically inequivalent ³¹P (I=1/2) nuclei (Figure 6). The X‐band EPR spectrum of (3‐As)GaCl₄ (g _iso=2.0539) at 298 K shows a multiplet with poorly resolved hyperfine components due to coupling of the unpaired electron with two magnetically inequivalent ⁷³As (I=3/2) nuclei (Figure 6). The magnitude of the hyperfine coupling constant (hfc) in (3‐P)GaCl₄ (A _iso(³¹P)=43 MHz) is comparable to those of a diphosphene–Cr radical cation (45 MHz) ^[15] and carbene‐stabilized radical cations (35–44 MHz). ^[32] The calculated hfc values at the B3LYP/def2‐TZVPP level of theory show a strong contribution of the P1 (196.5 MHz) and As1 (145.8 MHz) nuclei directly attached to the iron atom (Table S11 in the Supporting Information).

Figure 6.

EPR spectra of (3‐P)GaCl₄ and (3‐As)GaCl₄ (in THF, 298 K). Simulation parameters: g=2.0960, A _iso(³¹P) 311.9 MHz, A _iso(³¹P) 87.2 MHz, lw=2.44 mT [(3‐P)GaCl₄]; g=2.0539, A _iso(⁷⁵As) 424.6 MHz, A _iso(⁷⁵As) 82.7 MHz, A _iso(¹⁴N) 126.1 MHz, A _iso(¹⁴N) 50.0 MHz, lw=3.95 mT [(3‐As)GaCl₄].

In line with the experimental observation, calculations (Scheme S1 in the Supporting Information) support the preference of η³‐EC_vinylC_Ph bonding mode of the ligand in (4‐E)⁺² over the η³‐EEC_vinyl bonding style. The latter is preferred in (3‐E)^.+ and 2‐E. The isomeric model compounds [(4‐E)²⁺]^# featuring a similar η³‐EEC_vinyl coordination mode were found to be energetically less stable by 7.1 kcal mol⁻¹ (E=P) and 9.0 kcal mol⁻¹ (E=As) than the isolated examples (4‐E)(GaCl₄)₂ with the η³‐EC_vinylC_Ph bonding mode. The η³‐EEC_vinyl to η³‐EC_vinylC_Ph coordination shuttling is unfavorable for 2‐E (ΔG=37.5 kcal mol⁻¹, E=P; 36.1 kcal mol⁻¹, E=As) and (3‐E)^.+, (ΔG=19.4 kcal mol⁻¹, E=P; 18.0 kcal mol⁻¹, E=As; Scheme S1 in the Supporting Information). The HOMO of (4‐E)(GaCl₄)₂ (Figures S34 and S35 in the Supporting Information) is the σ‐type orbital mainly located at the E−E bond. The HOMO−1 is located at the C_vinyl−Fe, E−E, and E−Fe bonds with a small contribution from the phenyl substituent. The LUMO is mainly the antibonding C_IPr=C_vinyl bond with a significant contribution from a p‐type orbital of one of the pnictogen atoms. The NPA atomic partial charges for (4‐E)(GaCl₄)₂ (Table S10 in the Supporting Information) reveal a positive charge at the Fe(CO)₃ moiety of +0.35 e (E=P) and +0.33 e (E=As), indicating weaker donation from the ligand.

Further bonding insights were obtained by the EDA calculations. Donor–acceptor‐type interactions represent the best bonding description. Compared to 2‐E and (3‐E)GaCl₄, the interactions in (4‐E)(GaCl₄)₂ become more orbital stabilized (≈9 % dispersion, ≈37 % electrostatic, and ≈54 % orbital). In absolute values, the electrostatic interaction decreases by 38.0 kcal mol⁻¹ for (4‐P)(GaCl₄)₂ and 38.5 kcal mol⁻¹ for (4‐E)(GaCl₄)₂. The UV/Vis spectra of 2‐P (λ _max=373 and 459 nm) and 2‐As (λ _max=354 and 475 nm) exhibit two main absorptions (Figures S20 andS21 in the Supporting Information), which, on the basis of TD‐DFT calculations, can be assigned to H−1/H→L and H→L+1 transitions, respectively (Tables S17 and S18 in the Supporting Information; H=HOMO and L=LUMO). The UV/Vis spectra of (3‐E)GaCl₄ (Figures S22 and S23 in the Supporting Information) show two absorptions (λ _max=462 and 508 nm, E=P; 458 and 510 nm, E=As) in the visible region, which are related to S(α)→L and H(β)→L transitions, respectively (Tables S19 and S20 in the Supporting Information). ^[33] The UV/Vis absorption band at 518 nm (E=P) or 530 nm (E=As) for (4‐E)(GaCl₄)₂ (Figures S22 and S23 in the Supporting Information) is related to H→L/L+1 transitions (Tables S21 and S22 in the Supporting Information). The IR spectra of 2‐P (1876, 1909, 1972 cm⁻¹), (3‐P)GaCl₄ (1933, 1954, 2013 cm⁻¹), and (4‐P)(GaCl₄)₂ (2025, 2054, 2076 cm⁻¹) exhibit three bands for the CO stretching vibrations (Figures S14, S16, and S18 in the Supporting Information). A similar trend of increasing wavenumber can also be seen for the arsenic derivatives (Figures S15, S17, and S19 in the Supporting Information), which is consistent with depletion of the electron density at the iron atom on sequential 1e oxidation from 2‐E and (3‐E)GaCl₄, resulting in weaker dπ_Fe→π*_CO backbonding from 2‐E to (3‐E)GaCl₄ to (4‐E)(GaCl₄)₂.

Conclusion

We have reported the synthesis, structure, and redox chemistry of two Fe⁰ complexes 2‐E (E=P or As) featuring unconventional divinyldipnictene ligands as crystalline solids. The sequential 1e oxidation of 2‐E with GaCl₃ was shown to afford crystalline metalloradical cations (3‐E)GaCl₄ and dications (4‐E)(GaCl₄)₂. The formation of the latter occurs with unprecedented η³‐EEC_vinyl‐ to η³‐EC_vinylC_Ph‐coordination shuttling, which has been rationalized by theoretical calculations. Compounds (3‐E)GaCl₄ were characterized by cyclic voltammetry, EPR spectroscopy, UV/Vis spectroscopy, and single‐crystal X‐ray diffraction. The studies revealed that (3‐E)GaCl₄ are stabilized by delocalization of the unpaired electron over the CE₂Fe moiety, while the spin density is mainly located at the Fe and vinylic carbon atoms. The findings are expected to attract further interest in the isolation and exploration of other metalloradicals based on 1‐E.

Experimental Section

Deposition Numbers 1939894, 1998503, 1998504, 1998505, and 1998506 contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

M. K. Sharma, D. Rottschäfer, B. Neumann, H.-G. Stammler, S. Danés, D. M. Andrada, M. van Gastel, A. Hinz, R. S. Ghadwal, Chem. Eur. J. 2021, 27, 5803.