Ruthenium(II) and Ruthenium(III) Complexes of p-Benziporphyrin

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Ruthenium(II) and Ruthenium(III) Complexes of p‑Benziporphyrin: Merging Equatorial and Axial Organometallic Coordination Aneta Idec, Miłosz Pawlicki, and Lechosław Latos-Grazẏ ński* Department of Chemistry, University of Wrocław, 14 F. Joliot-Curie Street, 50-383 Wrocław, Poland S Supporting Information *

ABSTRACT: A diamagnetic ruthenium(II) complex of 5,10,15,20tetraphenyl-p-benziporphyrin [RuII(p-BzP)(CO)Cl] was obtained via the insertion of ruthenium into p-benziporphyrin using triruthenium(0) dodecacarbonyl [Ru3(CO)12] as the metal source. The procedure applying dichloro(cycloocta-1,5-diene)ruthenium(II) (polymer, [Ru(COD)Cl2]n) afforded the paramagnetic sixcoordinate ruthenium(III) p-benziporphyrin [RuIII(p-BzP)Cl2]. As shown by X-ray crystallography, the p-phenylene ring in both complexes is sharply tilted out of the N3 plane, as reflected by the respective N3 (pyrrole)−C6 (p-phenylene) dihedral angle [RuII(p-BzP)(CO)Cl, 52.5°; RuIII(p-BzP)Cl2, 53.7°]. p-Phenylene is bound to the ruthenium cation in an η2 fashion, revealing the shortest ever Ru−C distance in the series of p-benziporphyrin complexes [RuII(p-BzP)(CO)Cl, 2.275(2) Å; RuIII(p-BzP)Cl2, 2.324(5) Å]. The reaction of RuII(p-BzP)(CO)Cl with ArMgCl or AlkMgCl results in the formation of diamagnetic six-coordinate ruthenium(II) p-benziporphyrin complexes containing the apically coordinated σ-alkyl or σ-aryl ligands, where the metal ion simultaneously coordinates to three carbon centers respectively accommodating η2 (phenylene) and σ (aryl and alkyl) modes. Reactions of σ-aryl (alkyl) carbanions with paramagnetic RuIII(pBzP)Cl2 have been followed by 1H NMR spectroscopy. The procedure afforded the six-coordinate paramagnetic ruthenium(III) p-benziporphyrin [RuIII(p-BzP)(Ph)Cl], which binds one σ-aryl ligand, as reflected by the characteristic 1H NMR spectra spread within the +120 to −120 ppm range. Both paramagnetic complexes RuIII(p-BzP)(Ph)Cl and RuIII(p-BzP)(p-Tol)Cl are formed as a mixture of two stereoisomers differentiated by two nonequivalent locations of σ-aryl with respect to the puckered macrocyclic ring. The paramagnetic shifts of σ-aryls are indicative of π-spin delocalization patterns. Analysis of the contact shifts and parallel density functional theory calculations of the spin density distribution in RuIII(p-BzP)Cl2, RuIII(p-BzP)(Ar)Cl, and RuIII(p-BzP)(Alk)Cl reflect the features of the dxy2(dxzdyz)3 electronic ground state.

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INTRODUCTION

Typically, p-benziporphyrin acts as a monoanionic ligand coordinating through the three nitrogen donors; nevertheless, a specific construction of p-benziporphyrin imposes a structure promoting a side-on interaction with p-phenylene that results in a remarkable η2 interaction of the metal ion with the benzene ring.10,11,13,14 The incorporation of palladium(II), gold(III), or rhodium(III) into p-benziporphyrin changes the fundamental reactivity of the built-in p-phenylene moiety,13−15 generating a cascade of intramolecular rearrangements that are initiated by an interaction of arene and metal ion and eventually leading to respective complexes of 21-carbaporphyrin or its derivatives (Chart 2). As a part of our continuing program, aimed to investigate the organometallic chemistry of diamagnetic and paramagnetic metallocarbaporphyrinoids, here we report on the synthesis and spectroscopic characterization of ruthenium(II) and ruthenium(III) complexes of p-benziporphyrin. One has to be aware of the fact that the organometallic chemistry of paramagnetic metallocarbaporphyrinoids has been focused on a rather limited number of transition-metal ions [copper(II), iron(II,III), and

1−9

Carbaporphyrins, porphyrin analogues with a C−H unit in the coordination core, afforded rare organometallic compounds of transition elements that stabilize unusual metal oxidation states or uncommon coordination geometries involving frequently weak metal−carbon interaction. p-Benziporphyrin7,10−12 offers a unique opportunity to create a novel class of organometallic compounds explored for a set of transition metals [cadmium(II),10,11 zinc(II),11 nickel(II),11 palladium(II),13 and rhodium(III)14; Chart 1] and to control their reactivity.8,13−15 Chart 1. p-Benziporphyrin [(p-BzP)H] and Its Reported Coordination Modes

Received: May 14, 2017

© XXXX American Chemical Society

A

DOI: 10.1021/acs.inorgchem.7b01237 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

resolution mass spectrometry (HRMS). Calcd for C47H30N3ORu ([M − Cl]+): m/z 754.1425. Found: m/z 754.1432. The electronic absorption spectrum of RuII(pBzP)(CO)Cl in CH2Cl2 is shown in Figure 1. In contrast to

Chart 2. Rhodium(III) 5,10,15,20-Tetraphenyl-21methylene-21-carbaporphyrin14

nickel(I,II)]. The selected ligands such as N-confused porphyrin, O-confused porphyrin, m-benziporphyrin, N-confused pyriporphyrin, vacataporphyrin, ethyneporphyrin, and pbenziporphyrin provided the appropriate macrocyclic environment.1,16,17 Ruthenium(n) complexes of carbaporphyrins remain rarely explored, and the available motifs are limited to obvious equatorial ligation,18,19 restricting their stabilization mostly to ruthenium(II). Recently, we have reported on the open-shell ruthenium(II) azuliporphyrin π-cation radical containing the CNNN set of donors.19 The relevant heteroanalogue, 23thiaazuliporphyrin (CNSN), stabilized the alternative electronic state with a typical paramagnetic ruthenium(III) configuration.20 Interestingly, an azulene moiety incorporated into the macrocycle (carbaporphyrin or carbaporphyrinogen) provides also the π-surface suitable to bind the Ru4(CO)9 cluster.19,21 The short overview can be completed by atypical ruthenocenetype complexes of N-fused porphyrins22 and ruthenocenoporphyrin in which a d-electron subunit has been built into a πelectron conjugation pathway.23 In general, the organometallic paramagnetic ruthenium(III) porphyrins are rather rare and typically quite reactive, although with some remarkable exceptions.20,24,25 From that perspective, a controlled and predictable formation of ruthenium(II,III)···carbon interactions is of critical importance and opens new venues for exploration of the behavior of ruthenium porphyrins especially in the context of the extensive interest in their reactivity,26−37 motivated by the catalytic activity of such complexes where the formation of C−H, C−C, C−N, and CO bonds has been reported.30,36,38−40

Figure 1. UV−vis electronic spectra of (p-BzP)H (blue), RuII(pBzP)(CO)(n-Bu) (green), RuII(p-BzP)(CO)Cl (red), and RuII(pBzP)(CO)(Ph) (violet) in CH2Cl2.

p-benziporphyrin, which shows the Soret band at 433 nm and Q bands at 600 nm, the electronic absorption spectrum of RuII(p-BzP)(CO)Cl demonstrates a split Soret band at 416 and 513 nm and broad Q bands with maxima at 596 and 750 nm. The 1H NMR spectrum of RuII(p-BzP)(CO)Cl presented at Figure 2 shows the pattern of chemical shifts expected for diamagnetic complexes of p-benziporphyrin similar to previously reported ones.10,11,13,14 The spectrum contains two pphenylene signals (δ2,3 = 8.74 ppm, δ21,22= −0.5 ppm, 195 K) with no signs of conformational exchange accompanied by an AB spin system at δ = 8.91 ppm [H(7,18)] and δ = 8.69 ppm

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RESULTS AND DISCUSSION Preparation and Characterization of Diamagnetic Ruthenium(II) Complexes. Ruthenium(II) p-benziporphyrin [RuII(p-BzP)(CO)Cl] was obtained via the insertion of ruthenium into p-benziporphyrin performed in high-boiling solvents (e.g., o-dichlorobenzene) using triruthenium(0) dodecacarbonyl [Ru3(CO)12] as the ruthenium source (Scheme 1). The complex is stable as a solid and in solution. The composition of RuII(p-BzP)(CO)Cl was verified by highScheme 1. Synthesis of RuII(p-BzP)(CO)Cl

Figure 2. 1H NMR spectra of (A) RuII(p-BzP)(CO)Cl (195 K, CD2Cl2), (B) RuII(p-BzP)(CO)(Ph) (250 K, CDCl3), and (C) RuII(pBzP)(CO)(n-Bu) (230 K, CD2Cl2). In all traces, the resonance assignments of p-benziporphyrin (obtained from COSY and NOESY maps) follow the systematic numbering (Scheme 1). In trace B, oPh(Ru), m-Ph(Ru), and p-Ph(Ru) denote resonances of the σ-phenyl ring (red). In trace C, α-CH2, β-CH2, γ-CH2, and δ-CH3 are in blue. B


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Inorganic Chemistry

Crystal Structure of RuII(p-BzP)(CO)Cl. The geometry of RuII(p-BzP)(CO)Cl determined by X-ray diffraction is presented in Figure 3. The molecular structure of RuII(p-

[H(8,17)] with a coupling constant typical for a pyrrole ring in porphyrinoids (3J = 5.1 Hz) and a singlet at δ = 8.59 ppm assigned to H(12,13). Evidently, ruthenium(II) η 2-CC coordination (vide infra) prevents the libration of p-phenylene. The ruthenium(II)−p-phenylene interaction can be readily confirmed by the coordination shifts, i.e., the 13C NMR chemical shift difference determined between the C(21,22) resonances of RuII(p-BzP)(CO)Cl and p-benziporphyrin (Δδ = 45.7 ppm) or between C(2,3) and C(21,22) of RuII(pBzP)(CO)Cl (Δδ = 48.04 ppm). The presence of the carbonyl ligand was verified by the characteristic frequency of the CO stretching band [ν(CO) = 1959 cm−1] recorded with IR and 13C NMR spectroscopy with a chemical shift of 183.5 ppm typical for a carbonyl ligand (Figure S2 in the Supporting Information, SI). The reaction of RuII(p-BzP)(CO)Cl with ArMgCl or AlkMgCl in toluene results in the formation of six-coordinate ruthenium(II) p-benziporphyrin complexes containing the apically coordinated alkyl [RuII(p-BzP)(CO)(Alk) (Alk = Me, n-Bu)] or aryl [RuII(p-BzP)(CO)(Ar) (Ar = Ph, p-Tol)] ligands, where the metal ion coordinates to three carbon centers (Scheme 2). All new complexes present a high stability

Figure 3. Crystal structure of RuII(p-BzP)(CO)Cl (perspective view). Insets: (A) side view with aryl groups omitted for clarity; (B) geometry of the interaction between ruthenium(II) and the pphenylene moiety. Selected bond lengths [Å]: Ru−C(21) 2.274(2), Ru−C(22) 2.286(2), Ru−N(23) 2.152(6), Ru−N(24) 2.068(6), Ru− N(25) 2.152(6), Ru−Cl 2.463(18).

BzP)(CO)Cl demonstrates the predisposition of the ruthenium(II) center for octahedral geometry, in which the three nitrogen atoms and C(21)−C(22) bond occupy the equatorial locations, whereas the axial positions are occupied by the chloride anion and the carbonyl ligand. The ruthenium(II) ion is displaced from the N3 plane by ca. 0.076 Å toward the axial chloride. The molecular structure resembles some fundamental features of carbonylruthenium(II) porphyrin 4 8 or carbonylruthenium(II) azuliporphyrin,18 albeit the p-phenylene ring built into the macrocyclic system imposes the marked elongation of Ru−N(23) and Ru−N(25) bond distances in comparison to the typical values determined for ruthenium(II) porphyrin48 or to the Ru−N(24) bond of the same molecule. The orientation of the p-phenylene ring is reflected by the tilt angle of 52.5° with respect to the N3 plane, which is similar to the recently reported rhodium(III) p-benziporphyrin.14 The projection of the ruthenium ion onto the C(2)C(3)C(21)C(22) plane (C4 plane) lies close to the center of the C(21)− C(22) bond, so the metal ion interacts with the benzene ring in an η2 fashion. The distance between ruthenium and C(21) or C(22) [2.274(2) and 2.286(2) Å, respectively] is smaller than the expected for Ru−C van der Waals contact (ca. 4.21.Å)49 but in the range normally observed, for instance, for RuIV− C(allyl) bond lengths [2.303(5)−2.382(3)Å].50−53 The Ru− C(21,22) interaction for RuII(p-BzP)(CO)Cl is the shortest metal−carbon distance observed among the hitherto published complexes of p-benziporphyrin [RhIII(p-BzP)Cl2, 2.341(2) Å; NiII(p-BzP)Cl, 2.583(4) Å; CdII(p-BzP)Cl, 2.748(2) Å; PdII(pBzP)Cl, 2.831(2) Å].10,11,13,14 Ruthenium(III) p-Benziporphyrin. Any attempts to produce the carbonylruthenium(II) 5,10,15,20-tetraaryl-p-benziporphyrin π-cation radical [RuIII(p-BzP)(CO)Cl]•+ via oxidation with Br2 or 2,3-dichloro-5,6-dicyano-1,4-benzoquinone by analogy to previously reported carbonylruthenium(II) 5,10,15,20-tetraarylazuliporphyrin18 were ineffective. Nonetheless, the use of ruthenium sources without carbonyl ligands, i.e., dichloro(cycloocta-1,5-diene)ruthenium(II) (polymer), in odichlorobenzene afforded the one-electron-oxidized species, i.e., the six-coordinate paramagnetic ruthenium(III) complex

Scheme 2. Formation of RuII(p-BzP)(CO)(R) Complexes (R = Ph, p-Tol, Me, n-Bu)

and can be purified by typical chromatography techniques or recrystallization. The spectroscopic characterization (1H NMR, UV−vis, and HRMS; see the SI) in solution has confirmed an axial coordination of carbanion, where only chloride has been replaced by an aryl or alkyl anion. The mentioned axial coordination has significantly influenced the UV−vis electronic absorption spectra, as can be noticed in Figure 1 for RuII(p-BzP)(CO)(n-Bu) or RuII(p-BzP)(CO)(Ph). In fact, the pronounced change in the UV−vis electronic spectra due to carbanion coordination has been demonstrated because the characteristic split Soret band of RuII(p-BzP)(CO)Cl has been replaced by a broad but single one for both types of axially modified complexes. The 1H NMR spectra of RuII(p-BzP)(CO)(n-Bu) and RuII(p-BzP)(CO)(Ph) (Figure 2B,C) reveal the spectroscopic patterns typical for diamagnetic metalloporphyrins containing axially coordinated alkyl or aryl ligands.41−45 Evidently, they reflect the effective Cs symmetry imposed by equatorial pbenziporphyrin and a fast rotation or libration of the σ-alkyl or σ-aryl ligands with respect to Ru−Caryl or Ru−Calkyl bonds (Figure 2). The resonances expected for coordinated alkyl or aryl groups marked at Figure 2 are appropriately upfieldrelocated because of the ring current effect that is a characteristic feature of diamagnetic metalloporphyrin complexes bearing axial alkyl or aryl ligands.41,44−47 The characteristic multiplicity due to scalar coupling has been preserved. In addition, the incorporation of aryl/alkyl has been confirmed by HRMS (electrospray ionization, ESI). The presence of the remaining carbonyl in all complexes has been confirmed spectroscopically; i.e., 13C NMR spectra have shown a resonance characteristic for the CO unit (see the SI). C


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Inorganic Chemistry RuIII(p-BzP)Cl2 (Scheme 3). The composition was verified by HRMS. Calcd for C46H30ClN3Ru ([M − Cl]+): m/z 761.1172. Found: m/z 761.1169. Scheme 3. Synthesis of RuIII(p-BzP)Cl2

Figure 5. 1H NMR (CD2Cl2, 220 K) spectrum for RuIII(p-BzP)Cl2. In all traces, the resonance assignments of p-benziporphyrin follow the systematic numbering (see Scheme 1). Two sets of meso-phenyl resonances are labeled (o1, m1, and p1 and o2, m2, and p2, respectively).

Crystal Structure of RuIII(p-BzP)Cl2. The X-ray-determined molecular structure of RuIII(p-BzP)Cl2 (Figure 4) has

positions on each meso-aryl ring will be distinguishable because of the p-benziporphyrin nonplanarity. The broadened signal, which is shifted to δ = 54.1 ppm (300 K), is assigned to the inner H(21,22). Its position and line width reflect the proximity to the paramagnetic ruthenium(III) center. This argument was reinforced by a quantitative analysis of T1 relaxation times, which reflected approximate distances between the ruthenium(III) ion and the certain protons in the molecule.54,55 Paramagnetically induced relaxation may be of both dipolar and scalar origin, but the scalar contribution (which can be estimated from chemical shifts) is small in the studied compounds and has been omitted in the calculations. Relaxation times T1 is expressed simply as T1 = ar6, where r is the distance between the nucleus and a paramagnetic metal center, and a is a constant, which is equal for all of the protons in the molecule. The equation T1 = ar6 leads to a linear dependence between T1 and r in a bilogarithmic scale, with the slope equaling 6. Figure 6 (trace A) shows the T1 dependence on r for RuIII(p-BzP)Cl2. The r values are the averages of the crystal structure. The data for T1 and r are gathered in Table S1 in the SI. The resulting fit is fairly accurate especially with respect to the H(21,22), H(2,3), and pyrrolic β-H hydrogen atoms. In particular, the marked difference in the r values determined by H(21,22) and β-H hydrogen atoms allowed their straightforward differentiation based only on analysis of the T1 values. The scalar coupling between H(7) and H(8) hydrogens afforded the appropriate cross-peak in the COSY map of RuIII(p-BzP)Cl2. The remaining β-H resonance has been assigned by default to the H(12,13) hydrogen atoms. A Curie plot for 1H NMR resonances of RuIII(p-BzP)Cl2 is given in Figure 7. The experimental data are consistent with linear behavior over the whole temperature range. The extrapolated intercepts of p-phenylene, pyrrole, and mesophenyl resonances match the reference positions of diamagnetic RuII(p-BzP)(CO)Cl. Electronic Structure of RuIII(p-BzP)Cl2. 1H NMR spectroscopy was shown to be a uniquely definitive method for detecting and characterizing paramagnetic metalloporphyrinoids54−57 and can be applied as a definitive probe for detecting metal−arene interaction in benziporphyrins.11,16,56 It was shown that, depending on axial ligation, the one-electron oxidation of ruthenium porphyrins afforded ruthenium(II) porphyrin π-cation radicals or ruthenium(III) porphyrins.44,57−61 In the second case, two fundamental (dxy)2(dxzdyz)3 or less-common (dxy)1(dxzdyz)4 electronic structures are possible, distinguished by the distribution of an unpaired

Figure 4. Crystal structure of RuIII(p-BzP)Cl2 (perspective view). Insets: (A) side view with aryl groups omitted for clarity; (B) geometry of the interaction between ruthenium(III) and the pphenylene moiety. Selected bond lengths [Å]: Ru−C(21) 2.324(5), Ru−C(22) 2.334(5), Ru−N(23) 2.017(4), Ru−N(24) 2.129(4), Ru− N(25) 2.137(4), Ru−Cl(1) 2.330(14), Ru−Cl(2) 2.375(15).

structural features of RuII(p-BzP)(CO)Cl with slightly longer Ru−C(21) and Ru−C(22) bonds but still in the range normally observed for Ru−C bond lengths.50−53 The similar geometric appearance of paramagnetic RuIII(p-BzP)Cl2 and diamagnetic RuII(p-BzP)(CO)Cl (Figure 3) determined by Xray analysis implies a negligible influence of the replacement of the carbonyl group by the chloride ligand on the structural preferences. A noticeable shortening of the Ru−N bond lengths has also been noticed. The Ru−Cl bonds are also reduced [2.330(14)/ 2.375(15) Å vs 2.463(18) Å] for RuII(p-BzP)(CO)Cl, as expected for ruthenium(III) and the lack of a trans axial carbonyl ligand. The orientation of the p-phenylene ring is reflected by the tilt angle of 53.7° with respect to the N3 plane resembling the geometry of RuII(p-BzP)(CO)Cl. 1 H NMR Studies of Ruthenium(III) p-Benziporphyrin. The 1H NMR spectrum of paramagnetic RuIII(p-BzP)Cl2, corresponding to the effective Cs symmetry of the molecule, is consistent with the structure shown in Figure 5. The assignment of signals for RuIII(p-BzP)Cl2 was done on the basis of the relative intensities, line-width analysis, and measurements of the T1 relaxation times. An analogous approach has been used for all investigated paramagnetic species. Relatively long T1 relaxation times were afforded for the narrow signals of meso-aryl resonances, facilitating their assignments via NOESY and COSY experiments. One could easily separate the meso-aryl resonances into two scalar-coupled subsets, each assigned to a single meso-aryl unit. With the mesoaryl rings, it may be anticipated that the ortho and meta D


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Inorganic Chemistry

Figure 6. Relaxation plots for RuIII(p-BzP)Cl2 (A) and RuIII(p-BzP)(Ph)Cl (B). The dependence between T1 (CDCl3, 300 K) and r [the crystal structure averages for RuIII(p-BzP)Cl2 or averages of the distances derived from the DFT-optimized structure for RuIII(p-BzP)(Ph)Cl] is linear in bilogarithmic axes. Labels follow those presented in Figures 5 and 8.

electron on d orbitals to be directly correlated with the diagnostic pattern of paramagnetically shifted 1H NMR resonances. In classical terms, the pattern of the isotropic shift detected for Ru III (p-BzP)Cl 2 seems to be consistent with the dxy2(dxzdyz)3 metal electronic ground state, with a singly occupied dπ orbital interacting with a ligand π orbital, as found in a series of low-spin iron(III) porphyrins, iron(III) Nmethylporphyrin or ruthenium(III) porphyrin.44,46,62,63 The phenyl resonances (ortho, meta, and para) of RuIII(p-BzP)Cl2 demonstrate clearly the alternation of meso-aryl isotropic shifts m-H downfield and o-H and p-H upfield), which is consistent with the dominating contribution of the contact shift combined with the negligible input of the dipolar part.62 The upfield-shifted resonances reflect the positive spin density at all β-H positions. The presence of the p-phenylene moiety instead of the fourth five-membered ring allows the observation of very specific spin delocalization distinctive for this class of carbaporphyrin in which the p-phenylene resonances demonstrate the consistently downfield contact shift [H(21,22), 54.1 ppm; H(2,3), 12.6 ppm; 300 K]. Reactivity RuIII(p-BzP)Cl2 with Aryl Grignard Reagents. The reaction of metalloporphyrins with Grignard reagents is a classical synthetic method in organometallic chemistry introducing the σ-aryl/alkyl apical ligands. As presented

Figure 7. Curie plot for the p-phenylene [(21,22) and (2,3); blue], pyrrole [(7,18/8,17) and (12,13); green], and meso-phenyl (m2, p1, and o2; orange) resonances of RuIII(p-BzP)Cl2. The experimental data are consistent with linear behavior over the whole temperature range. Labels follow those presented in Figure 5.

Scheme 4. Formation of RuIII(p-BzP)(Ar)Cl Complexes (Ar = Ph, p-Tol)a

a

Stereoisomers syn and anti are shown for RuIII(p-BzP)(Ar)Cl. E


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Inorganic Chemistry

Figure 8. 1H NMR (CD2Cl2, 300 K) for (A) RuIII(p-BzP)(Ph)Cl and (B and C) RuIII(p-BzP)(p-Tol)Cl. The resonance assignments of pbenziporphyrin follow the systematic position numbering, o-H-s, m-H-s, and p-H-s/p-Me, and o-H-a, m-H-a, and p-H-s/p-Me-a denote resonances of the σ-aryl ring assigned to RuIII(p-BzP)(Ar)Cl-syn and RuIII(p-BzP)(Ar)Cl-anti stereoisomers. Inset C: Zoomed region containing p-Me resonances. with the spectrum collected at the 200 to −100 ppm range. For clarity, the 0−15 ppm regions of all spectra are omitted.

above, the reaction of RuII(p-BzP)(CO)Cl with Grignard reagents leads to organometallic compounds where the σcarbanion is bound axially as documented spectroscopically. Consequently, we have decided to explore the analogous chemistry for the paramagnetic RuIII(p-BzP)Cl2. A reaction of RuIII(p-BzP)Cl2 in toluene-d8 with phenylmagnesium chloride or p-tolylmagnesium chloride monitored by 1H NMR resulted in the formation of diamagnetic RuII(pBzP)(Ph)THF or RuII(p-BzP)(p-Tol)THF (Scheme 4 and Figure S22 and S23 in the SI), proving a dual role of the Grignard reagent, which acts as a one-electron reducing agent and, subsequently, as a source of σ-aryl.47,64−67 The presence of tetrahydrofuran (THF) in the investigated solution causes pronounced changes. Understanding these is particularly important because the ruthenium(II,III) σ-aryl or σalkyl complexes are initially prepared in the presence of THF (the solvent for the Grignard reagent), and our preliminary examination of the titration of RuIII(p-BzP)Cl2 with the aryl Grignard reagent was complicated by a combination of two factors: the ruthenium(III) reduction and the solvent composition.68 A transient paramagnetic [RuIII(p-BzP)(Cl)THF]+ species prone to one-electron reduction has been identified in the course of titration with PhMgCl monitored by the 1H NMR, where the recorded spectral pattern reveals the characteristic features resembling RuIII(p-BzP)Cl2 and confirms the axial coordination of the solvent because the four downfield-shifted resonances characteristic for THF have been observed (Figure S27a in the SI). The substitution of a chloride ligand with THF seems to be necessary to allow the reduction step to form diamagnetic RuII(p-BzP)(Cl)THF (Figure S27b in the SI). The subsequent addition of ArMgCl in excess immediately yields RuII(p-BzP)(Ph)THF, consistent with the suggested reaction pathway. The reaction with (pTol)MgCl seems to be similar, albeit the intermediary paramagnetic species has not been directly detected. The 1H NMR spectra of RuII(p-BzP)(Ph)THF and RuII(p-BzP)(pTol)THF (Figures S22 and S23 in the SI) closely resemble that

of RuII(p-BzP)(CO)(Ph) or RuII(p-BzP)(CO)(p-Tol). The patterns are typical for diamagnetic complexes of porphyrins with axially coordinated appropriate σ-aryl ligands, reflecting the effective Cs symmetry of molecules with characteristically upfield-relocated resonances of the o-H, m-H, and p-H (p-Me) of the aryl directly bound to ruthenium(II). The presence of a single σ-aryl has been confirmed by the detailed analysis of integrations. Titration of RuII(p-BzP)(Ph)THF or RuII(p-BzP)(p-Tol)THF solutions in dichloromethane-d2 with Cl2 at 300 K afforded the six-coordinate paramagnetic ruthenium(III) pbenziporphyrin RuIII(p-BzP)(Ar)Cl, which binds one σ-aryl ligand and one chloride, as documented in the 1H NMR spectra observed in the remarkably wide range (+120 to −120 ppm, 300 K; Figure 8). The prolonged exposure of RuII(pBzP)(Ph)THF or RuII(p-BzP)(p-Tol)THF on dioxygen in toluene-d8 or dichloromethane-d2 leads to one-electron oxidation as well. Eventually, it was established that column chromatography (silica gel) in aerobic conditions immediately afforded RuIII(p-BzP)(Ph)Cl or RuIII(p-BzP)(p-Tol)Cl. The isolated species have been characterized by UV−vis electronic spectroscopy and their compositions confirmed by HRMS (see the SI). Actually, detailed analysis of the 1H NMR spectra of paramagnetic RuIII(p-BzP)(Ph)Cl or RuIII(p-BzP)(p-Tol)Cl revealed the characteristic spectroscopic features consistent with the presence of two complete sets of resonances (Figure 8). Thus, RuIII(p-BzP)(Ar)Cl is actually a mixture of two stereoisomers that could not be separated by column chromatography. They will be denoted as RuIII(p-BzP)(Ar)Cl-anti and RuIII(p-BzP)(Ar)Cl-syn, as shown in Scheme 4 and illustrated by the appropriate density functional theory (DFT) models in Figure 9. In the five-coordinate nickel(II), cadmium(II), and palladium(II) p-benziporphyrin complexes NiII(p-BzP)Cl, CdII(p-BzP)Cl, and PdII(p-BzP)Cl and six-coordinate rhodium(III) p-benziporphyrin RhIII(p-BzP)Cl2, conformation of the F


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Inorganic Chemistry

straightforward differentiation based only on analysis of the T1 values. Figure 10 shows a plot of the temperature dependence of the 1 H NMR chemical shifts for RuIII(p-BzP)(Ph)Cl-anti and RuIII(p-BzP)(Ph)Cl-syn, i.e., typical examples of complexes with σ-aryl ligands. Both show similar behavior for the pyrrole resonances, which seems characteristic of low-spin ruthenium(III) p-benziporphyrins. However, the Curie plots for aryl resonances are evidently different. It is apparent that, in general. they do not extrapolate to positions of the diamagnetic reference [RuII(p-BzP)(CO)(Ph)]. Presumably, thermal equilibrium involving σ-aryl libration provides the reason for the observed deviations because each rotamer is expected to be characterized by different sets of contact shifts. The electronic ground state of ruthenium(III) of RuIII(pBzP)Cl2 seems to be preserved in RuIII(p-BzP)(Ar)Cl because the fundamental patterns of its “fingerprint” resonance (β-H pyrrole and p-phenylene) are significantly similar. Consistently, it includes the characteristic alternation of contact shifts seen for meso-aryls, marked downfield shifts of inner H(21,22) resonances, and upfield relocation of β-H pyrrole resonances. The remarkably large isotropic shifts of axially coordinated σaryl or σ-alkyl reflect practically entirely the contact component. The macrocyclic contact shift values are markedly different for each isomer. Thus, they are evidently determined by the location of σ-aryl with respect to macrocyclic folding. For aryl anions coordinated to ruthenium(III), the alternating sign pattern of the aryl-H contact shift (o-H and p-H negative, m-H positive, and p-Me positive) is indicative of mainly π-spin delocalization at the aryl group. As expected for π-spin delocalization, the contact shifts for protons and methyl groups in the para position are of opposite sign but similar magnitude.69,70 In these complexes, the aryliron(III) (S = 1/2) or arylruthenium(III) moiety are low-spin d5 systems with a (dxy)2(dxz,dyz)3 electronic configuration. As a consequence, the aryl−ruthenium(III) bond is formed by a σ interaction between the (nominally unoccupied) ruthenium(III) dz2 orbital and the highest occupied orbital of the aryl fragment having a large amplitude on the Cipso orbital and a π interaction between the (nominally half occupied) ruthenium(III) dxz,yz orbitals and the occupied π MOs of the σ-aryl unit.

Figure 9. DFT-optimized models of RuIII(p-BzP)(Ph)Cl-anti (A and B) and RuIII(p-BzP)(Ph)Cl-syn (C and D).

macrocycle reflects the phenylene C(2)/C(3) rim tilted away, simultaneously pushing the internal C(21)/C(22) rim to interact with a central metal. The fifth coordination position is occupied by the axially bound chloride in the anti orientation, as confirmed by the respective X-ray structures.10,11,13 In the case of NiII(p-BzP)Cl, it was demonstrated that a minute amount of the syn conformer was present in solution and rapidly equilibrated with the respective anti form, significantly influencing variation of the line widths in 1H NMR spectra.12 Consequently, the 1H NMR spectral patterns of RuIII(pBzP)(Ar)Cl reflect the isomerism resulting from two nonequivalent orientations of the axial aryl with respect to the puckered macrocyclic ring. The major spectroscopic features determined by the electronic ground state remain identical for both stereoisomers, which allowed the specific assignment of characteristic resonances. In fact, the fundamental assignments can be made by a comparison with those of RuIII(p-BzP)Cl2 and resemble those of RuIII(TPP)Ph.24 Figure 6 (trace B) shows the T1 dependence on r for RuIII(p-BzP)(Ph)Cl-anti and RuIII(p-BzP)(Ph)Cl-syn. The r values are the averages of distances derived from the DFT-optimized structures (Figure 9). The resulting fit is fairly accurate especially with respect to the σ-aryl and H(21,22) hydrogen atoms, which allowed their

Figure 10. Temperature dependence of the chemical shifts for p-phenylene (21,22), pyrrole (12,13), and σ-phenyl (m-H, p-H, and o-H) of RuIII(pBzP)(Ph)Cl-anti and RuIII(p-BzP)(Ph)Cl-syn in CD2Cl2. The solid lines show extrapolation of the experimental data points as expected from the Curie law. Labels follow those presented in Figure 8. G


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Inorganic Chemistry The pattern of σ-aryl resonances reported for RuIII(pBzP)(Ar)Cl resembles the basic features reported for the aryl anions axially bound to low-spin iron(III,IV) porphyrins71−74 or ruthenium(III) porphyrins.32,33 In those cases, the analogous binding mode accounting for the π-spin density distribution was invoked. A similar approach has been considered to explain the spin distribution in the phenyl ligand of the low-spin iron(IV) coordinated to a closed-shell corrolate trianion and to a phenyl monoanion.75 DFT calculations (to be discussed below) yield alternating signs at the o-, m-, and p-carbon atoms, with similar magnitudes of the spin densities, consistent with π-spin delocalization. At further considerations and assignments, we have presumed that the stereoisomer RuIII(p-BzP)(Ar)Cl-anti reveals the larger absolute value isotropic shifts at o-H and p-Me σ-aryls consistent with the theoretical predictions (vide infra). Two scenarios are feasible here. Both forms are expected to remain in the thermodynamic equilibrium. Alternatively, if the activation barrier of structural rearrangement is too high, they are simultaneously formed in the course of alkylation or arylation, but the feasible conversion rate is too slow to achieve thermodynamic equilibrium in the explored temperature range. The conformational rearrangement was expected to generate a systematic variation of the molar ratio for stereoisomers RuIII(pBzP)(Ar)Cl-anti and RuIII(p-BzP)(Ar)Cl-syn as a function of the temperature. The relative amounts of syn and anti stereoisomers, evaluated by integration of the appropriate 1H NMR signals assigned to two different species in 1H NMR, seem to be constant in the whole temperature range, however, in the limits of the large error of integration predictable for essentially large line widths of the involved resonances. This observation seems to exclude the conformational rearrangements where the p-phenylene of p-benziporphyrin exchanges between two nonplanar conformations RuIII(p-BzP)(Ar)Cl-anti and RuIII(p-BzP)(Ar)Cl-syn in a seesaw fashion characteristic for five-coordinate nickel(II) complexes.12 The relatively strong RuIII···η2-C2 p-phenylene interaction seems to be instrumental in the stabilization of both conformers. Significantly, no evidence for analogous rearrangement has been noticed for RuII(p-BzP)(CO)Cl because separated p-phenylene H(2,3) and H(21,22) resonances were observed in the whole temperature region. Reactivity of RuIII(p-BzP)Cl2 with Alkyl Grignard Reagents. The addition of AlkMgCl in excess to RuIII(pBzP)Cl2 immediately yielded diamagnetic RuII(p-BzP)Alk, consistent with the suggested reaction pathway (Scheme 5).

upfield region41−43 (Figures S24 and S26 in the SI). The spectroscopic patterns, consistent with the formation of a single isomer, were confirmed by NOE contacts seen in the NOESY map for Me−H(21,22) hydrogen atoms, detectable solely for RuII(p-BzP)(Alk)-syn (Figure S25 in the SI). The subsequent oxidation with Cl2 afforded paramagnetic (RuIII(p-BzP)(Me)Cl and RuIII(p-BzP)(n-Bu)Cl) complexes. The effect of oxidation is shown in Figure 11. The spectral

Figure 11. 1H NMR (CD2Cl2, 300 K) for (A) RuIII(p-BzP)Cl2, (B) RuIII(p-BzP)(Me)Cl, and (C) RuIII(p-BzP)(n-Bu)Cl. The insets present resonances of the coordinated alkyl: (D) Me; (E) α-CH2.

patterns are those expected for paramagnetic ruthenium(III) pbenziporphyrin including RuIII(p-BzP)Cl2 and RuIII(p-BzP)(Ar) Cl discussed in detail above with some variations of the contact shifts due to differences in that axial ligations. The spectroscopic patterns are consistent with the formation of a single species (arbitrarily demonstrated RuIII(p-BzP)(n-Bu)Clanti instead of two feasible stereoisomers). The coordination of alkyl has been unambiguously confirmed by the detection of far-downfield-located resonances assigned to methyl or αmethylene groups of the respective RuIII(p-BzP)(Alk)Cl complexes: RuIII(p-BzP)(Me)Cl, 650.0 ppm; RuIII(p-BzP)(nBu)Cl, 651.0 ppm in 300 K (CD2Cl2). These resonance positions are consistent with the (dxy)2(dxz,dyz)3 electronic ground state of ruthenium(III). The similar extremely downfield position for α-CH resonances of a coordinated σ-alkyl ligand was previously reported for low-spin ethyliron(III) porphyrin.76 In spite of the intensive search, the β-CH2 resonances of RuIII(p-BzP)(n-Bu)Cl could not be detected

Scheme 5. Formation of RuIII(p-BzP)(Alk)Cl Complexes (Alk = Me, n-Bu)

The 1H NMR spectra of RuII(p-BzP)(Alk)THF complexes closely resemble those of RuII(p-BzP)(CO)Alk. The patterns seem to be typical for diamagnetic complexes of porphyrins with axially coordinated appropriate σ-alkyl ligands and reflect the effective Cs symmetry of the molecules. The integrations indicate that only one σ-alkyl ligand is present. The alkyl resonances of RuII(p-BzP)(Alk)THF are located in the typical H


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Inorganic Chemistry and are presumably hidden in the crowded region. This remains in contrast with alkyliron(III) porphyrin, where the β-CH2 resonances have been readily identified in the upfield spectral region.76 In a classical description, the Ru−C σ bond is derived entirely from the filled α-carbon-based sp3 hybrid orbital of alkyl and a vacant dz2 orbital on the ruthenium(III) ion. Because the ruthenium(III) ion possesses an unpaired electron on dxzdyz orbitals, the correlation favors the transfer of the positive spin density, leaving a net negative spin density on an α-carbon atom. The further spin redistribution involves the appropriate molecular orbital of the alkyl, rendering, however, a positive spin density on the α-hydrogen atoms and affording the remarkable downfield contact shift. DFT Calculations: Molecular Structures. The principal geometries of diamagnetic RuII(p-BzP)(CO)Cl, RuII(p-BzP)(CO)(Ph), RuII(p-BzP)(CO)(p-Tol), RuII(p-BzP)(CO)(Me), and RuII(p-BzP)(CO)(n-Bu) and paramagnetic RuIII(p-BzP)Cl2, RuIII(p-BzP)(Ph)Cl-syn, RuIII(p-BzP)(Ph)Cl-anti, RuIII(pBzP)(p-Tol)Cl-syn, RuIII(p-BzP)(p-Tol)Cl-anti, RuIII(p-BzP)(Me)Cl-syn, RuIII(p-BzP)(Me)Cl-anti, RuIII(p-BzP)(n-Bu)Clsyn, and RuIII(p-BzP)(n-Bu)Cl-anti were subjected to DFT optimizations. The final geometries are shown in Figures 9 and S29−S37 in the SI. In each case, a genuine energy minimum was obtained. Significantly, the DFT-optimized structural parameters of RuII(p-BzP)(CO)Cl-anti and RuIII(p-BzP)Cl2 are essentially similar to those determined by X-ray crystallography, which adds credibility to the geometry of the remaining DFT-optimized structures. In the series of ruthenium(II) and ruthenium(III) p-benziporphyrin complexes, the variations in the bond lengths were rather negligible, and the determined values resembled those found for RuII(pBzP)(CO)Cl and RuIII(p-BzP)Cl2, respectively. In particular, the Ru−C(21) bond lengths in the optimized structures are ca. 0.1 Å longer than those determined by X-ray crystallography {RuII(p-BzP)(CO)Cl [DFT, 2.368 Å; X-ray determined, 2.274(2) Å] and RuIII(p-BzP)Cl2 [DFT, 2.447 Å; X-ray determined, 2.324(5) Å]} but still in the Ru−C bond length limits (see above). Typically, the energy difference between the two stereoisomers RuIII(p-BzP)(R)Cl-anti and RuIII(p-BzP)(R)Cl-syn is in the range 2−4.5 kcal/mol (p-Tol, 2.12 kcal/mol; Me, 2.14 kcal/mol; Ph, 3.06 kcal/mol; n-Bu, 4.27 kcal/mol). The rather small values are consistent with the simultaneous formation of both stereoisomers in the course of the reaction of ArMgCl with RuIII(p-BzP)Cl2, as evidenced by their unambiguous identification by 1H NMR spectroscopy. The 1H NMR chemical shifts of diamagnetic complexes have been calculated using the GIAO−B3LYP method for all optimized structures of diamagnetic complexes. Satisfactory linear correlations between the calculated and experimental values of the chemical shifts for each complex have been determined (Figures S38−S42 in the SI). DFT Calculations: Electronic Structure and Spin Density (Contact Shift) Analysis. To assess the electronic structure of paramagnetic ruthenium(III) p-benziporphyrin, population analysis was performed. Figure 12 presents a diagram of selected molecular orbitals for RuIII(p-BzP)Cl2 that are representative for all investigated systems, although their relative energetic positions vary as a function of the axial ligations. In general, the electronic ground states of RuIII(pBzP)Cl2, RuIII(p-BzP)(Ar)Cl, and RuIII(p-BzP)(Alk)Cl correspond to the experimentally derived description of the ruthenium(III) porphyrin structure with a (dxy)2(dxz)2(dyz)1 electronic configuration.

Figure 12. Diagram of selected energy levels and relevant molecular orbitals of RuIII(p-BzP)Cl2 for α and β electrons separately. All surfaces mapped with an isovalue of 0.02 (e/au3)1/2 (positive maxima marked in red).

Analysis of the molecular orbitals diagram for RuIII(pBzP)Cl2 (Figure 12) provides insight into the specific contributions of dxz and dyz ruthenium(III) orbitals in lowest unoccupied molecular orbital (LUMO), highest occupied molecular orbital (HOMO), HOMO−1, HOMO−2, and HOMO−3 (singly occupied molecular orbital, SOMO) of RuIII(p-BzP)Cl2. The electronic structures of RuIII(p-BzP)(Alk) Cl and RuIII(p-BzP)(Ar)Cl, as determined by DFT, bear essentially the similar fundamental features evidently modulated by the choice and position of the alkyl and aryl ligands with respect to the equatorial ligand folding. Evidently, the molecular orbitals preserve the essential spatial patterns discussed already for RuIII(p-BzP)Cl2 even if their relative energies vary. The detailed population analysis allowed the identification of the SOMO. Significantly for RuIII(p-BzP)Cl2, SOMO = HOMO−3, mostly participating in the spin density distribution (vide infra), has a lower energy than HOMO, HOMO−1, and HOMO−2 occupied by both α and β electrons. Such SOMO and HOMO level switching is a known phenomenon that can be induced and was also experimentally observed for porphyrin complexes.77−79 For all complexes, as exemplified at Figure 12, the SOMO contains the significant component of the metal dyz orbital. Furthermore, the composition of SOMO indicates that I


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Inorganic Chemistry this orbital significantly covers the p-phenylene and transpyrrole fragments, resembling the eπ orbital in D4h symmetry of regular porphyrins. The pattern of the spin density, as determined for the electronic ground state, is shown in Figure 13. This readily

Figure 14. Schematic representation of (A) the experimental paramagnetic and (B) calculated contact shift patterns (1H NMR) for RuIII(p-BzP)Cl2. Color code: blue, pyrrole; red, p-phenylene; green, meso-phenyl.

comparison, the 1H NMR data have been plotted solely for the electronic ground state, avoiding the arbitrary evaluation of the excited-state population. It is important to emphasize that DFT calculations reproduced qualitatively the essential spectroscopic features including the marked downfield shift of H(21,22) pphenylene signals and π delocalization at meso-aryls. Assuming that a rotation (or libration) barrier of the axial aryl with respect to the Ru−Cipso bond in RuIII(p-BzP)(Ar)Cl is very low, the twisting of the aryl may significantly perturb the electronic state for a given aryl position, causing significant changes of the spin density distribution as well. A specific different distribution of the spin density is expected, as it is for any analogous metalloporphyrin systems containing an axial planar ligand with a π-interaction capability. Previously, it was demonstrated that occupation of the d orbitals can be dictated by the orientation of the planar axial ligands. For instance, such behavior was explored experimentally and theoretically for lowspin bis(imidazole)iron(III) porphyrins, which typically acquire the (dxy)2(dyz,dxz)3 electronic configuration. It was clearly demonstrated that a small perturbation such as rotation of the imidazoles with respect to the Fe−N(Im) axis was enough to change the occupation of the dπ orbitals and consistently the spin density distribution at the equatorial porphyrin.69,70,85−87 The spin density distribution as determined for the ground state of RuIII(p-BzP)(Ph)Cl is shown in Figure 15. In particular, one is expecting the π-density pattern at the axially coordinated phenyl ligand. In reality, the picture in RuIII(p-BzP)(Ar)Cl is quite complex and reflects the population average spin density characteristic for each essential rotamer of a RuIII-aryl unit. Eventually, we have decided to relate the experimentally determined contact shifts (and related spin density distribution) to the limiting case of the electronic state (ground or excited), favoring SOMO (see Table S19 in the SI). Such a choice seems to be consistent with the remarkably large contact shifts of H(21,22) resonances detected in each investigated complex, which are solely consistent with such an electronic structure. As the 1 H NMR data show, the features corresponding to the effective Cs symmetry for all investigated species are consistent with fast rotation or libration engaging evidently asymmetric forms. Accordingly, the average spin density values for all dynamically related positions have been considered. At this part of analysis, the axial σ-aryl and σ-alkyl ligands have been treated as suitable spectroscopic probes that provide independent theoretical and spectroscopic insight into the electronic structure of σ-aryl- and σ-alkylruthenium(III) pbenziporphyrins. Considering the complexity of their 1H NMR spectra, further analysis has been focused on the markedly relocated resonances of axial ligand, trans-pyrrole, and pphenylene.

Figure 13. Plot of the RuIII(p-BzP)Cl2 molecular orbital [A; isovalue = 0.02 (e/au3)1/2; positive maxima marked with red] and total spin density surfaces for the electronic ground state (B; isovalue = 0.0004 e/au3; positive density marked with blue).

noticed experimentally detected spin density distribution, as reflected by the experimentally determined contact shifts of RuIII(p-BzP)Cl2 in 1H NMR spectra, can be defined dominantly by the SOMO of the electronic ground state. Alternatively, assuming that the energy gap between the HOMO and LUMO is sufficiently small, the model has to include some contribution from the Boltzmann population of the thermally accessible excited state(s).69,70,80,81 In general, the electronic states examined for these RuIII(pBzP)(X)Cl complexes are all rather close in energy. Thus, on the basis of the DFT calculations alone, it would be difficult to make an unequivocal prediction about the actual experimental electronic structure and conformations. The spectroscopic data and calculations together, however, allow one to identify a preferred electronic state for these complexes. Spin densities determined in the course of DFT studies on organic radicals or paramagnetic metalloporphyrins can be compared with experimental values: isotropic hyperfine coupling constants aN and contact shifts obtained from electron paramagnetic resonance (EPR) and NMR spectroscopic measurements.69 The spin density on carbon or hydrogen atoms (Table S19 in the SI) can be converted into the isotropic shifts of attached protons. The detailed discussion of the methodology was given previously.82−84 A comparison of the experimental and theoretical spectra for RuIII(p-BzP)Cl2 is schematically presented in Figure 14. Analysis leads to the fundamental conclusion that at least a qualitatively spin distribution of RuIII(p-BzP)Cl2 is reasonably well accounted for by the spin density calculations. To get a better qualitative match between the isotropic shift pattern of the H(7) and H(8) resonances and the DFT-determined spin density patterns, one has to resort to the population averaged spin density values ρobs = p(EGS) ρ(EGS) + p(EES) ρ(EES), which may allocate the additional spin density at cis-pyrroles. Still, to simplify the J


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Inorganic Chemistry

Figure 15. Plot of the RuIII(p-BzP)(Ph)Cl-anti total spin density surfaces for the electronic ground state (with isovalues = 0.0004 e/au3). The positive density is marked in blue. The perpendicular projections demonstrate the alternation of the π spin density at the σ-phenyl ligand.

Figure 16. Schematic representation of the contact shift patterns for RuIII(p-BzP)(Ph)Cl-anti [(A) experimental and (B) calculated] and RuIII(pBzP)(Ph)Cl-syn [(C) experimental and (D) calculated]. Color code: blue, pyrrole; red, p-phenylene; green, axial phenyl.

delocalization (spin densities included in Table S19 in the SI in parentheses in the contact shift column). The expected reversal of the spin density signs for the p-H and p-Me groups (σ-Ph vs σ-Tolyl) has been reproduced as well (Figures 15 and S47 and S48 in the SI). One can notice that the pattern of σ-Ph spin densities of RuIII(p-BzP)(Ph)Cl-anti is clearly reproduced by RuIII(p-BzP)(Ph)Cl-syn, but all spin density values are simply scaled down 2.3 times. In the case of RuIII(p-BzP)(Alk)Cl, the negative spin density has been located at the α-carbon atom and positive at the αhydrogen atom, consistent with the 1H NMR pattern. In particular, the enormous contact shift of the σ-Me ligand (650 ppm) has been albeit qualitatively reproduced (300 ppm). A comparison of the experimental and theoretical spectra is schematically presented in Figures S44 and S45 in the SI). Analysis leads to the fundamental conclusion that the spin distributions are qualitatively well accounted for axial ligands by the ruthenium(III) porphyrin structures.

The selected chemical shifts, contact shifts, carbon and hydrogen spin density values, and calculated contact shifts for representative RuIII(p-BzP)(Ar)Cl-anti and RuIII(p-BzP)(Ar)Cl-syn complexes are given in Table S19 and Figure S43 in the SI. In the case of σ-aryl complexes, distinct differences between the measured contact shifts have been observed for RuIII(pBzP)(Ar)Cl-anti and RuIII(p-BzP)(Ar)Cl-syn (Figure 16). Still, the available experimental data do not provide any information for their distinction. Arbitrarily, the marked difference in the determined experimentally contact shift value for σ-aryl of RuIII(p-BzP)(Ar)Cl-anti and RuIII(p-BzP)(Ar)Cl-syn has been taken advantage of. The following relationships hold between the contact shifts of the two stereoisomers: |δcon(Ho)anti| ≫ |δcon(Ho)syn|; |δcon(Hm)anti| > |δcon(Hm)syn|; |δcon(Hp)anti| ≫ |δcon(Hp)syn|; |δcon(p-Me)anti| ≫ |δcon(p-Me)syn|. Analysis of the spin density reproduces these relationships, assuming that the larger absolute value contact shifts at the o-H, m-H, and p-H σ-aryl positions are characteristic of RuIII(p-BzP)(Ar)Cl-anti. In the case of σ-alkyl complexes, the single isomer was typically detected, so its tentative identification was not feasible. Eventually, the results of the calculations were given for both structural options (Table S19 in the SI). Importantly, DFT calculations yielded remarkably large and alternating signs for the spin densities at the o-, m-, and p-carbon atoms, with similar magnitudes of the spin densities consistent with the π-spin

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CONCLUSIONS In this contribution, we synthesized and explored the specific coordination properties of p-benziporphyrin, which coordinates ruthenium(II) and ruthenium(III) as a monoanionic ligand binding in the equatorial plane to three pyrrole nitrogen and pphenylene carbon atoms. The p-phenylene ring is tilted out of that plane and is bound to the metal in an η2 fashion, forming K


Article

Inorganic Chemistry

2H, 3J = 7.4 Hz), 5.76 (t, 1H, 3J = 7.3 Hz), 5.52 (t, 2H, 3J = 7.3 Hz), 2.65 (d, 2H, 3J = 7.7 Hz), −1.09 (s, 2H). 13C NMR (125.7 MHz, CDCl3, 300 K): δ 181.1, 167.9, 157.8, 154.7, 149.5, 147.4, 142.6, 142.0, 139.6, 139.5, 137.9, 136.8, 135.4, 133.9, 133.8, 133.1, 131.3, 130.4, 130.2, 129.8, 129.2, 129.1, 128.1, 126.8, 126.4, 126.3, 125.7, 125.3, 124.9, 124.1, 77.7. HRMS (ESI). Calcd for C52H35N3Ru ([M − CO]+): m/z 803.1902. Found: m/z 803.1876. UV−vis [CH2Cl2, 298 K; λmax, nm (log ε, M−1 cm−1)]: 424 (4.66), 564 (4.12), 731 (3.81). RuII(p-BzP)(CO)(p-Tol). RuII(p-BzP)(CO)Cl (10 mg, 0.0127 mmol) dissolved in dry toluene (10 mL) and a p-tolylmagnesium chloride solution (250 μL; 1.0 M in THF) was added. The resulting mixture was chromatographed on silica gel (dichloromethane), and the first brown fraction was collected, giving RuII(p-BzP)(CO)(p-Tol) with an isolated yield of 80.3% (8.6 mg). 1H NMR (600.15 MHz, 220 K, CDCl3): δ 9.07 (d, 2H, 3J = 7.6 Hz), 8.79 (d, 2H, 3J = 4.9 Hz), 8.61 (d, 2H, 3J = 4.9 Hz), 8.58 (s, 2H), 8.14 (s, 2H), 8.06 (d, 2H, 3J = 7.3 Hz), 7.94 (t, 2H, 3J = 7.6 Hz), 7.91 (d, 2H, 3J = 7.3 Hz), 7.67 (m, 6H), 7.61 (t, 2H, 3J = 7.5 Hz), 7.67 (t, 2H, 3J = 7.6 Hz), 7.16 (m, 2H), 5.32 (d, 2H, 3J = 8.1 Hz), 2.51 (d, 2H, 3J = 8.1 Hz), 1.39 (s, 3H), −1.16 (s, 2H). 13C NMR (125.7 MHz, CDCl3, 300 K): δ 180.8, 163.7, 158.1, 154.9, 149.7, 147.4, 142.8, 139.9, 139.6, 136.9, 133.2, 131.3, 130.3, 127.8, 126.6, 126.4, 125.9, 125.3, 125.1, 78.1, 20.3. HRMS (ESI). Calcd for C53H37N3Ru ([M − CO]+): m/z 817.2031. Found: m/z 817.2032. UV−vis (CH2Cl2, 298 K; λmax, nm): 420.0, 578.9, 721.3. RuII(p-BzP)(CO)(Me). RuII(p-BzP)(CO)Cl (5 mg, 0.00634 mmol) dissolved in dry toluene (10 mL) and a methylmagnesium chloride solution (100 μL; 2.0 M in THF) was added. The resulting mixture was chromatographed on silica gel (dichloromethane), and the first green fraction was collected, giving RuII(p-BzP)(CO)(Me) with an isolated yield of 65.7% (3.2 mg). 1H NMR (600.15 MHz, 250 K, CD2Cl2): δ 9.11 (d, 2H, 3J = 7.6 Hz), 8.63 (d, 2H, 3J = 5.0 Hz), 8.62 (s, 2H), 8.49 (d, 2H, 3J = 5.0 Hz), 8.39 (s, 2H), 8.12 (d, 2H, 3J = 7.5 Hz), 8.03 (d, 2H, 3J = 7.5 Hz), 7.95 (t, 2 H, 3J = 7.6 Hz), 7.69 (m, 8H), 7.56 (t, 2H, 3J = 7.6 Hz), 7.13 (t, 2H, 3J = 8.1 Hz), −1.4 (s, 2H), −5.61 (s, 3H).13C NMR (125.7 MHz, CDCl3, 300 K): δ 176.9, 158.1, 153.9, 148.7, 147.1, 142.7, 139.4, 138.2, 136.9, 133.7, 133.5, 133.4, 133.4, 131.2, 129.0, 128.4, 128.2, 128.0, 127.6, 126.7, 126.4, 126.3, 124.7, 77.5, 22.7. HRMS (ESI). Calcd for C47H30N3ORu ([M − CH3]+): m/z 754.14257. Found: m/z 754.1410. UV−vis [CH2Cl2, 298 K; λmax, nm]: 415.0, 503.9, 579.0. RuII(p-BzP)(CO)(n-Bu). RuII(p-BzP)(CO)Cl (10 mg, 0.0127 mmol) dissolved in dry toluene (10 mL) and a butylmagnesium chloride solution (200 μL; 2.0 M in THF) was added. The resulting mixture was chromatographed on silica gel (dichloromethane), and the first green fraction was collected, giving RuII(p-BzP)(CO)(n-Bu) with an isolated yield of 85.6% (8.8 mg). 1H NMR (600.15 MHz, 230 K, CD2Cl2): δ 9.01 (d, 2H, 3J = 7.6 Hz), 8.52 (d, 2H, 3J = 5.0 Hz), 8.48 (s, 2H), 8.36 (d, 2H, 3J = 5.0 Hz), 8.27 (s, 2H), 8.0 (d, 2H, 3J = 7.3 Hz), 7.90 (d, 2H, 3J = 7.5 Hz), 7.86 (t, 2H, 3J = 7.8 Hz), 7.60 (m, 8H), 7.48 (t, 2H, 3J = 7.5 Hz), 7.03 (d, 2H, 3J = 7.6 Hz), −0.39 (t, 2H, 3J = 7.5 Hz), −0.83 (m, 2H), −1.54 (s, 2H), −3.10 (m, 2H), −5.07 (m, 2H). 13C NMR (125.7 MHz, CDCl3, 300 K): δ 176.3, 158.2, 154.5, 149.0, 147.5, 142.9, 139.7, 138.6, 137.0, 133.8, 133.7, 133.5, 133.4, 131.3, 128.4, 128.2, 127.8, 126.6, 126.5, 125.0, 77.9, 30.0, 29.1, 26.4, 13.3. HRMS (ESI). Calcd for C50H39N3Ru ([M − CO]+): m/z 783.2187. Found: m/z 783.2185. UV−vis [CH2Cl2, 298 K; λmax, nm (log ε, M−1 cm−1)]: 425 (4.40), 507 (4.15), 592 (3.74), 647 (3.70), 751 (3.53). RuIII(p-BzP)(Ph)Cl. RuIII(p-BzP)Cl2 (5 mg, 0.00628 mmol) dissolved in dry toluene (10 mL) and a phenylmagnesium chloride solution (100 μL; 2.0 M in THF) was added in an inert atmosphere and resulted in the formation of a RuII(p-BzP)(Ph)THF intermediate. The prolonged exposure of RuII(p-BzP)(Ph)THF on dioxygen in toluene or dichloromethane leads to one-electron oxidation as well. Eventually, it was established that column chromatography (silicagel, ethyl acetate) in aerobic conditions immediately afforded RuIII(p-BzP)(Ph)Cl. The isolated species was obtained with an isolated yield of 78.9%. 1 H NMR (600.15 MHz, 300 K, CD2Cl2): δ 110.0, 82.60, 42.50, 22.4, 13.06, −15.20, −29.80, −42.40, −93.80, −107.30. HRMS (ESI). Calcd

the shortest ever Ru−C distance in the series of pbenziporphyrin complexes [RuII(p-BzP)(CO)Cl, 2.275(2) Å; RuIII(p-BzP)Cl2, 2.324(2) Å]. Here we have shown the paramagnetic organometallic complexes with chemical properties very different from those of regular ruthenium porphyrin, allowing for the simultaneous coordination of carbon atoms using η2 and σ modes. This work broadened our understanding of the paramagnetic metallocarbaporphyrinoids, exploring specific aspects of the electronic structure, in particular, the spin density distribution in equatorial (p-benziporphyrin) and axial (alkyl and aryl) ligands.

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EXPERIMENTAL SECTION

General Procedures. Chemicals and solvents like o-dichlorobenzene, hexane isomers, etc., were at least pure grade and used without purification. Dichloromethane was distilled over CaH2. [Ru (COD)Cl2]n was synthesized from RuCl3 and 1,5-cyclooctadiene.88 5,10,15,20-Tetraphenyl-p-benziporphyrin. 1,4-Bis[phenyl(2pyrolyl)methyl]benzene (541 mg, 1.4 mmol), pyrrole (98 μL, 1.4 mmol), and benzaldehyde (286 μL, 2.8 mmol) were added to dry dichloromethane (900 mL) under nitrogen. Boron trifluoride diethyl etherate (140 μL) was then added, and the reaction mixture was protected from light and stirred for 2 h. 2,3-Dichloro-5,6-dicyano-1,4benzoquinone (954 mg, 4.2 mmol) was subsequently added, and the reaction mixture was stirred for another 0.5 h. After that time, the solvent was evaporated under reduced pressure and the dark residue was subjected to chromatography [alumina (GII), dichloromethane]. The desired product was eluted as a green band following a trace of H2TPP. The crude product was recrystallized from dichloromethane/ methanol. Yield: 66 mg (7%). Analytical data are in agreement with those published for (p-BzP)H.10 RuII(p-BzP)(CO)Cl. p-Benziporphyrin (10 mg, 0.016 mmol) and Ru3(CO)12 (30.69 mg, 0.048 mmol) dissolved in o-dichlorobenzene (30 mL) were allowed to stir under reflux for 3 h. After that time, the solvent was evaporated under reduced pressure and the dark brown residue was subjected to chromatography (silica gel, dichloromethane), giving 8.3 mg (65.8%) of eluted ethyl acetate as the second fraction with a dark-brown color. 1H NMR (600.15 MHz, 195 K, CD2Cl2): δ 9.22 (d, 2H, 3J = 7.8 Hz), 8.91 (d, 2H, 3J = 5.1 Hz), 8.74 (s, 2H), 8.69 (d, 2H, 3J = 5.1 Hz), 8.59 (s, 2H), 8.08 (d, 2H, 3J = 8.3 Hz), 8.05 (d, 3J = 8.4 Hz), 7.99 (t, 2H, 3J = 7.6 Hz), 7.71 (m, 6H), 7. 61 (t, 3J = 7.5 Hz), 7.18 (d, 2H, 3J = 7.6 Hz), −0.5 (s, 2H). 13C NMR (125.7 MHz, CDCl3, 300 K): δ 183.5, 157.4, 153.2, 150.3, 147.5, 142.0, 139.8, 139.2, 137.3, 134.7, 134.1, 133.9, 132.6, 128.8, 128.4, 127.9, 126.7, 126.4, 125.1, 84.4. HRMS (ESI). Calcd for C47H30N3ORu ([M − Cl]+): m/z 754.1425. Found: m/z 754.1432. UV−vis [CH2Cl2, 298 K; λmax, nm (log ε, M−1 cm−1)]: 415.9 (4.47), 511.9 (4.49), 649 (3.97), 743 (3.85). IR (νCO): 1959 cm−1. RuIII(p-BzP)Cl2. p-Benziporphyrin (10 mg, 0.016 mmol) and [Ru(COD)Cl2]n (44.83 mg, 0.16 mmol) dissolved in o-dichlorobenzene (30 mL) were allowed to stir under reflux for 24 h. After that time, the solvent was evaporated under reduced pressure and the darkbrown residue was subjected to chromatography (silica gel, dichloromethane) and eluted as the first brown fraction. Yield: 7.1 mg (55.7%). 1 H NMR (600.15 MHz, 230 K, CD2Cl2): δ 70.30, 14.15, 12.68, 10.50, 10.30, 8.79, 3.99, 3.58, −1.72, −1.60, −2.00, −4.92, −6.37, −7.90, −20.01. HRMS (ESI). Calcd for C46H30ClN3Ru ([M − Cl]+): m/z 761.1172. Found: m/z 761.1169. RuII(p-BzP)(CO)(Ph). RuII(p-BzP)(CO)Cl (5 mg, 0.00634 mmol) dissolved in dry toluene (10 mL) and a phenylmagnesium chloride solution (100 μL; 2.0 M in THF) was added in an inert atmosphere. The resulting mixture was chromatographed on silica gel (dichloromethane), and the first brown fraction was collected, giving RuII(pBzP)(CO)Ph with an isolated yield of 77.9% (4.1 mg). 1H NMR (600.15 MHz, 250 K, CDCl3): δ 9.06 (d, 2H, 3J = 7.4 Hz), 8.77 (d, 2H, 3J = 5.0 Hz), 8.6 (d, 2H, 3J = 5.0 Hz), 8.58 (s, 2H), 8.12 (s, 2H), 8.07 (d, 2H, 3J = 7.3 Hz), 7.92 (m, 2H), 7.90 (d, 2H, 3J = 7.3 Hz), 7.61 (m, 16H), 7.45 (t, 4H, 3J = 7.6 Hz), 7.35 (t, 2H, 3J = 7.3 Hz), 7.13 (d, L


Article

Inorganic Chemistry for C52H35N3Ru ([M − CO]+): m/z 803.1902. Found: m/z 803.1873. UV−vis (CH2Cl2, 298 K; λmax, nm): 443.9, 530.1, 605.1, 663.4. RuIII(p-BzP)(p-Tol)Cl. RuIII(p-BzP)Cl2 (5 mg, 0.00628 mmol) dissolved in dry toluene (10 mL) and a p-tolylmagnesium chloride solution (250 μL) (1.0 M in THF) was added in an inert atmosphere and resulted in the formation of a Ru II (p-BzP)(p-Tol)THF intermediate. The prolonged exposure of RuII(p-BzP)(p-Tol)THF on dioxygen in toluene or dichloromethane leads to one-electron oxidation as well. Eventually, it was established that column chromatography (silicagel, ethyl acetate) in aerobic conditions immediately afforded RuIII(p-BzP)(p-Tol)Cl. The isolated species was obtained with an isolated yield of 78.1%. 1H NMR (600.15 MHz, 300 K, CD2Cl2): δ 148.64, 114.91, 80.69, 60.87, 41.80, 22.30, −12.20, −16.30, −46.60, −119.94. HRMS (ESI). Calcd for C53H37N3Ru ([M − CO]+): m/z 817.2031. Found: m/z 817.2030. UV−vis (CH2Cl2, 298 K; λmax, nm): 444.9, 570.3, 668.3. RuIII(p-BzP)(Me)Cl. RuIII(p-BzP)Cl2 (5 mg, 0.00628 mmol) dissolved in dry toluene (10 mL) and a methylmagnesium chloride solution (100 μL; 2.0 M in THF) was added in an inert atmosphere and resulted in the formation of a RuII(p-BzP)(Me)THF intermediate. The prolonged exposure of RuII(p-BzP)(Me)THF on dioxygen in toluene or dichloromethane leads to one-electron oxidation as well. Eventually, it was established that column chromatography (silicagel, ethyl acetate) in aerobic conditions immediately afforded RuIII(pBzP)(Me)Cl. The isolated species was obtained with an isolated yield of 56.4%. 1H NMR (600.15 MHz, 270 K, CD2Cl2): δ 647.00, 102.44, 18.81, 11.12, 8.41, 7.03, 6.24, 5.40, 2.50, 1.06, 0.80, −1.11, −1.63, −7.10, −19.86. HRMS (ESI). Calcd for C46H30N3ClRu ([M − CH3]+): m/z 761.1172. Found: m/z 761.1169 9. UV−vis (CH2Cl2, 298 K; λmax, nm): 441.9, 555.0, 600.8, 673.1. RuIII(p-BzP)(n-Bu)Cl. RuIII(p-BzP)Cl2 (5 mg, 0,00628 mmol) dissolved in dry toluene (10 mL) and a butylmagnesium chloride solution (200 μL; 2.0 M in THF) was added in an inert atmosphere and resulted in the formation of a Ru II (p-BzP)(n-Bu)THF intermediate. The prolonged exposure of RuII(p-BzP)(n-Bu)THF on dioxygen in toluene or dichloromethane leads to one-electron oxidation as well. Eventually, it was established that column chromatography (silicagel, ethyl acetate) in aerobic conditions immediately afforded RuIII(p-BzP)(n-Bu)Cl. The isolated species was obtained with an isolated yield of 59.8%. 1H NMR (600.15, MHz 300 K, CD2Cl2): δ 660.21, 100.64, 22.47, 15.87, 12.72, 10.49, 8.08, 7.43, 6.09, 6.02, 5.3, 2.56, −1.35, −2.76, −9.65, −21.92. HRMS (ESI). Calcd for C50H39N3Ru ([M + H]+): m/z 819.1976. Found: m/z 819.1948. UV−vis (CH2Cl2, 298 K; λmax, nm): 441.0, 549.0, 666.9. NMR Spectroscopy. NMR spectra were measured on Bruker Avance 500 MHz and Bruker Avance III 600 MHz spectrometers. 1H and 13C shifts were referenced to the residual resonances of deuterated solvents. Paramagnetically shifted spectra were recorded with fast recycle times (usually below 100 ms). The maximum spectral width allowed by the hardware was ca. 200 ppm for 1H, so that the extreme low-field signals were recorded in separate runs. Longitudinal relaxation times (T 1 ) were determined using the standard inversion−recovery technique. The recovery time was varied from 1 μs to 1 s in an approximately logarithmic fashion in order to cover the broad range of decays. Mass Spectrometry. High-resolution and accurate mass spectra were recorded on Bruker Apex Ultra FTMS and Bruker micro-TOF-Q spectrometers using the electrospray technique. UV−Vis Spectroscopy. Electronic spectra were recorded on a Varian Cary-50 Bio spectrophotometer. Crystallography. The X-ray diffraction data for RuII(p-BzP)(CO) Cl and RuIII(p-BzP)Cl2 were collected on Oxford Diffraction Xcalibur with a Ruby detector (Mo Kα radiation; λ = 0.71073 Å). Data reduction and analysis were carried out with the CrysAlis Pro program.89 Structures were solved by direct methods using the SHELXS program and refined using all F2 data, as implemented by the SHELXL-2013 program.90 The SQUEEZE procedure,91 implemented in the PLATON program, was applied for disorder too severe to be modeled lattice solvent molecules.

DFT Calculations. Geometry optimizations were carried out within unconstrained C1 symmetry in vacuo, with starting coordinates derived from preoptimized models or crystal structures using Gaussian 09 software.92 Harmonic frequencies were calculated using analytical second derivatives as a verification of the local minimum achievement with no negative frequencies observed. In ligand studies, calculations were performed at the B3LYP/6-31G(d,p)93,94 level of theory. NMR shifts were calculated using the GIAO method with tetramethylsilane shieldings as a reference for NMR. Metal complexes were optimized using the B3LYP functional (see the SI for details) with a 6-31G(d,p) basis set and pseudopotential (LANL2DZ) on ruthenium. Paramagnetic ruthenium(III) species were optimized using a spinunrestricted approach. The first excited-state geometry was obtained from time-dependent DFT optimalization. Subsequent population analyses gave spin density distributions for both configurational isomers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01237. Additional 1H and 13C NMR and HRMS spectra, crystallographic data, structural analysis, DFT-optimized models with relative energies, calculated 1H NMR correlations, and DFT coordinates for all models (PDF) Accession Codes

CCDC 1549466 and 1549514 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Homepage: http://llg.chem.uni.wroc.pl/. ORCID

Lechosław Latos-Grażyński: 0000-0003-1230-9075 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Science Centre (Grant 2012/04/A/ST5/00593) is kindly acknowledged. DFT calculations have been carried out using resources provided by the Wroclaw Centre for Networking and Supercomputing (http:// wcss.pl).

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REFERENCES

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