Generation and Tunable Cyclization of Formamidinate Ligands in

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Generation and Tunable Cyclization of Formamidinate Ligands in Carbonyl Complexes of Mn(I): An Experimental and Theoretical Study Javier Ruiz,*,† Daniel Sol,† Lucía García,†,# María A. Mateo,† Marilín Vivanco,† and Juan F. Van der Maelen*,‡ Departamento de Química Orgánica e Inorgánica and ‡Departamento de Química Física y Analítica, Facultad de Química, Universidad de Oviedo, E-33006 Oviedo, Spain

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†

S Supporting Information *

ABSTRACT: A subtle change in the substituents at the nitrogen atoms in the N,N′-diarylformamidine complexes of formula fac-[Mn(RNC(H)NHR)(bipy)(CO)3]+ (1a, R = phenyl; 1b, R = 4-dimethylaminophenyl) produces, upon deprotonation, either the monodentate formamidinate complex fac-[Mn(PhNC(H)NPh)(bipy)(CO)3] (2a) or metallacylic complex 3b, which features a carbamoyl residue arising from nucleophilic attack to a vicinal carbonyl ligand. Complexes type 3 are also formed when using N-aryl-N′-alkyl-formamidines as well as N,N′-dialkylformamidines. Quantum Theory of Atoms in Molecules computations show the existence of a weak bond critical point of mainly noncovalent electrostatic type between the uncoordinated nitrogen atom of the formamidinate and a carbonyl ligand in 2a, which appears to be an isolable intermediate species that precedes the formation of metallacyclic complexes 3.

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INTRODUCTION

ligands, which have emerged as promising alternatives to widely used N-heterocyclic carbenes.8 The inclusion of an additional functional group led to hybrid amidinates which form metal complexes with different structural motifs and potential application in catalysis.9 Moreover, certain lanthanide amidinates are promising precursors for atomic layer deposition and metal−organic chemical vapor deposition processes in materials science.10 A point of additional interest in the case of formamidinate complexes is the possible postfunctionalization of the central carbon atom, as we have recently demonstrated with the generation of unique metalla-N-heterocyclic carbenes.11 The chelating and bridging coordination modes of formamidinates are very common in transition metal chemistry.1 In contrast, the monodentate coordination mode is rarely encountered in the literature, in spite of the potential rich reactivity that could emerge from the free nitrogen atom as a basic center.12 In this

Amidinate anions have been extensively used as ligands in metal complexes all along the periodic table, from transition metals to rare-earth and main group elements.1,2 They feature electronic and steric properties, as well as versatility and ready accessibility, to be considered as an alternative to cyclopentadienyl-based ligands to stabilize metal complexes in organometallic chemistry, coordination chemistry, and catalysis. Group 4 amidinate complexes have been utilized for achieving homogeneous postmetallocene olefin polymerization,3 and this application has been progressively extended to other transition metals and different polymerization processes.4 Combination of both cyclopentadienyl and amidinate ligands has also been used to properly achieve some catalytic reactions.5 The coordinative versatility of alkali-metal amidinates and their ability to affect a wide range of synthetic applications has been highlighted,6 whereas remarkable multiply bonded dinuclear complexes are known to be stabilized by bridging amidinate anions.7 Another interesting application of amidinates is the stabilization of heavy tetrylene © XXXX American Chemical Society

Received: December 12, 2018

A

DOI: 10.1021/acs.organomet.8b00898 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics regard, here we describe new manganese(I) complexes bearing formamidinate ligands featuring a rather infrequent monodentate coordination mode, which show a reminiscent intramolecular frustrated Lewis pair behavior that explain its propensity to undergo a cyclization process by nucleophilic attack to a vicinal carbonyl ligand. Theoretical calculations that send light on the nature of the interactions of the amidinate ligand with the carbonyl and bipyridine ancillary ligands are also included.

Scheme 2. Formation of Monodentate Formamidine Complexes 1c−g and Corresponding Metallacyclic Derivatives 3c−g

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RESULTS AND DISCUSSION Formation and Cyclization Reactions of Formamidinates. The generation of formamidinate complexes of Mn(I) begins with the synthesis of the corresponding formamidine precursors. This has been accomplished by two different approaches. N,N′-Diarylformamidine complexes of formula fac-[Mn(RNC(H)NHR)(bipy)(CO)3]+ (1a, R = phenyl; 1b, R = 4-dimethylaminophenyl) have been prepared by reaction of fac-[MnBr(bipy)(CO)3] with the appropriate formamidine in the presence of TlPF6 as bromide abstractor (Scheme 1). N-Aryl-N′-alkyl-formamidine complexes facScheme 1. Formation of Monodentate Formamidinate Complex 2a and Metallacyclic Derivative 3b produces after deprotonation the immediate cyclization of first generated formamidinate yielding complex 3b (Scheme 1), which was isolated as black crystals. In this case, a nucleophilic attack of the uncoordinated N-aryl moiety to a vicinal carbonyl ligand produces the five-membered metallacyclic compound 3b, featuring a carbamoyl residue. Apparently the slightly stronger donor character of the N-4-dimethyaminophenyl group with respect to the N-phenyl group induces the cyclization process in the last case. The N-aryl-N′-alkylformamidine complexes also affords metallacyclic compounds 3 after deprotonation, as well as the N,N′-dialkylformamidine complexes (Scheme 2). In both cases the corresponding formamidinates, containing an N-alkyl terminal group, are highly basic and immediately attack the vicinal carbonyl ligand to give the observed products. New compounds 1a, 1b, 2a, and 3b−g were characterized by spectroscopic methods. Additionally, the structure of 2a, 3b, and 3g was confirmed by an X-ray study. Curiously, even though complexes 1c−e were isolated as a mixture of isomers resulting from the formation of two formamidine tautomers, leading to coordination through the NMe group (showed in Scheme 2) or through the NR group (see ref 14), only one isomer of 3c−e, containing the carbamoyl moiety −NMeC O, was formed. The IR spectrum of 2a showed in the νCO region a characteristic pattern of bands for a fac-tricarbonyl complex, but at lower frequencies than those of cationic precursor 1a, which is in accordance with the formation of a monodentate formamidinate. None the less, complexes 3 feature IR spectra consisting of two intense νCO bands typical of cis-dicarbonyl complexes, thus confirming the cyclization process of the first generated formamidinates. In the 1H NMR spectra, the signal corresponding to the NCHN proton of the formamidine in 1a (7.75 ppm) is maintained essentially at the same frequency in monodentate formamidinate complex 2a (7.72 ppm), whereas that of 1b (7.50 ppm) is shifted downfield in metallacyclic derivative 3b (7.91 ppm). In the 13 C{1H} NMR spectra of 2a, the signal of the NCHN carbon atom appears at 161.1 ppm, whereas in metallacyclic

[Mn(RNC(H)NHMe)(bipy)(CO)3]+ (1c, R = phenyl; 1d, R = 2-naphthyl; 1e, R = 4-methoxyphenyl) as well as N,N′-dialkylformamidine complexes fac-[Mn(MeNC(H)NHR)(bipy)(CO)3]+ (1f, R = methyl; 1g, R = benzyl) have been synthesized by Ag2O-induced tautomerization of the corresponding diaminocarbene derivatives,13 following an experimental procedure recently described by our group (Scheme 2).14,15 Subsequently, a deprotonation reaction of cationic complexes type 1 to generate formamidinate derivatives was easily achieved by treatment with bases such as KOH or LiHMDS. In the case of the N,N’-diphenylformamidine the neutral complex fac-[Mn(PhNC(H)NPh)(bipy)(CO)3] (2a, Scheme 1) bearing a monodentate formamidinate ligand was formed as a stable species, which was isolated as an orange solid. A subtle change in the electronic properties of the formamidine on going from N,N′-diphenylformamidine to N,N′-bis(4-dimethylaminophenyl)formamidine (complex 1b) B

DOI: 10.1021/acs.organomet.8b00898 Organometallics XXXX, XXX, XXX−XXX

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Organometallics complexes type 3, this signal appears slightly shifted upfield (157−159 ppm). Complexes type 3 showed a dynamic behavior in solution depending on the temperature and the nature of the substituents at the nitrogen atoms of the metallacycle. Thus, for complex 3b containing a N,N′diarylformamidino moiety, only four groups of signals for the bipy ligand were present in the 1H NMR spectrum at room temperature, instead of the expected eight considering the absence of a symmetry plane in the molecule. Most probably, this behavior arises from a reversible transfer of the N-aryl group from the carbamoyl moiety to the vicinal carbonyl ligand, passing through monodentate formamidinate intermediate 2b (similar to 2a), thus rendering equivalent both halves of the bipy ligand. This dynamic process was not frozen even by reducing the temperature to 193 K. In the case of Naryl-N′-alkyl derivatives 3c−e, the 1H NMR spectra at room temperature also showed the above equivalence of the bipy hydrogen atoms, but the signals are split up at low temperature (193 K), showing that the dynamic process is frozen. The 13 C{1H} NMR spectra of 3c−e are also in accordance with this observation because just a low-field signal at around 226 ppm (corresponding to the axial carbonyl group) is observed at room temperature, whereas at 193 K the signals of the carbamoyl group (around 247 ppm) and the two inequivalent carbonyl ligands (225 and 226 ppm) were present. Finally, the 1 H NMR spectra of N,N′-dialkyl derivatives 3f and 3g displayed differentiated signals for the bipy ligand even at room temperature showing a higher stability of the metallacycle in this case. The formation of monodentate formamidinate complex 2a and the substituent-dependent dynamic behavior of complexes type 3 correlate well with the basicity of the formamidinate ligand, which increases in the order N,N′-diphenyl < N,N′bis(4-dimethylaminophenyl) < N-aryl-N′-alkyl < N,N′-dialkyl. Views of the X-ray structures of 2a, 3b, and 3g, together with selected bond distances and angles, are shown in Figures 1, 2, and 3, respectively. The structure of 2a clearly shows the monodentante formamidinate ligand bonded to manganese, featuring similar values of the N1−C2 (1.337(3) Å) and N3−C2 (1.300(3) Å) bond distances, which are intermediate between single and

Figure 2. View of the structure of 3b, shown with 30% thermal ellipsoids. Hydrogen atoms (except C2−H2) are omitted for clarity. Selected bond distances (Å) and angles (deg): Mn1−N1 = 2.054(6), N1−C2 = 1.284(9), C2−N3 = 1.349(9), N3−C4 = 1.482(9), C4− O4 = 1.233(8); N1−C2−N3 = 120.5(7), N1−Mn1−C4 = 81.0(3).

Figure 3. View of the structure of 3g, shown with 30% thermal ellipsoids. Hydrogen atoms of the bipy ligand and phenyl group are omitted for clarity. Selected bond distances (Å) and angles (deg): Mn1−N1 = 2.043(5), N1−C2 = 1.289(8), C2−N3 = 1.333(8), N3− C4 = 1.454(7), C4−O4 = 1.247(7); N1−C2−N3 = 118.7(5), N1− Mn1−C4 = 80.5(2).

double bond. Very interestingly, the uncoordinated N3−Ph moiety is oriented toward the equatorial plane of the coordination octahedral holding the bipy ligand. In fact, the distance from the nitrogen atom N3 to the carbon atom C4, belonging to an equatorial carbonyl ligand, of 2.940(3) Å seems to indicate the existence of a very weak noncovalent interaction between the free NPh fragment of the formamidinate and the carbonyl ligand, which appears to precede the formation of a real covalent N−C bond existing in the carbamoyl group of complexes type 3, an interaction that we studied in depth by DFT calculations as described bellow. Also worth noting is the N3−C20 distance (2.958(3) Å), suggesting the existence of an additional weak noncovalent interaction between the formamidinate and the bipy ligand. The X-ray structure of 3b (Figure 2) and 3g (Figure 3) confirmed the formation of the new diazametallacycle, showing N3−C4 distances typical of single bond (1.482(9) and 1.454(7) Å for 3b and 3g, respectively). There is a small difference between the C2−N1 (1.284(9) and 1.289(8) Å) and C2−N3 (1.349(9) and 1.333(8) Å) bond distances in the formamidino fragment, but these still indicate the existence of multiple bond character in the NCN skeleton. The small N1−Mn1−C4 bite-angle (around 81°) reflects the strain generated in the metallacycle,

Figure 1. View of the structure of 2a, shown with 30% thermal ellipsoids. Hydrogen atoms (except C2−H2) are omitted for clarity. Selected bond distances (Å) and angles (deg): Mn1−N1 = 2.0861(19), N1−C2 = 1.337(3), C2−N3 = 1.300(3), N3−C4 = 2.940(3), N3−C20 = 2.958(3); N1−C2−N3 = 125.2(2), N1−Mn1− C4 = 95.9(2). C

DOI: 10.1021/acs.organomet.8b00898 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 4. Molecular graph of 2a (experimental geometry) (a), 2a (theoretically optimized geometry in gas phase: see text for details) (b), 3b (experimental geometry) (c), and 3b (theoretically optimized geometry in gas phase) (d), showing bcp’s (small red spheres) and rcp’s (small yellow spheres), as well as bp’s (thin lines) and labeling scheme.

when compared with the value of that angle in acyclic formamidinate derivative 2a (95.9(2)°). As expected, complex 2a is immediately transformed to 1a by treatment with acids, and this is also true in the reaction of complexes type 3 with acids, showing that the formation of the formamidinate and metallacyclic derivatives is totally reversible. Theoretical Calculations. The images shown in Figure 4 were obtained by applying the QTAIM approach to compounds 2a and 3b.16 They show, along with the atoms corresponding to each complex, the complete set of bond critical points (bcp’s) and ring critical points (rcp’s), together with the bond paths (bp’s) that connect bonded atoms through their corresponding bcp’s. Very interestingly, as clearly seen in Figure 4a, which refers to the experimental geometry of complex 2a, bcp’s and bp’s were found between N3 atom and both C4 and C20 atoms, whereas for the theoretically optimized geometry of this complex (Figure 4b) as well as for both geometries of complex 3b (Figure 4c,d) only the N3− C4 bcp and bp were found. It is also of interest that the rather large curvatures shown by both N3−C4 and N3−C20 bp’s are near carbon atoms in Figure 4a, while all other bp’s in the four figures are nearly straight lines, which means that bond path lengths and interatomic distances are approximately equal for the latter but quite different for the former (see below for further discussion of this point). Figure 5 displays gradient trajectory maps of the total electron density in the N1−C2−N3 plane of complexes 2a and 3b, showing the atomic basins of Mn1, N1, C2, H2, and N3

Figure 5. Gradient trajectories mapped on total electron density plots (contour levels at 0.1 e Å−3) for the experimental geometry of 2a (top) and 3b (bottom), showing the atomic basins, bp’s (red lines), bcp’s (red circles), and rcp (green circle). D

DOI: 10.1021/acs.organomet.8b00898 Organometallics XXXX, XXX, XXX−XXX

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Organometallics atoms, as well as those of ligand atoms contained in the same plane. All bp’s, bcp’s, and rcp’s located in this plane can also be observed. As expected, C4 and O4 atomic basins can also be seen for complex 3b (Figure 5. bottom), due to the fact that the bridging carbonyl group in this complex belongs to the planar cycle Mn1−N1−C2−N3−C4. Interestingly enough, the same is true for the theoretically optimized geometry of complex 2a (see the Supporting Information), while on the contrary neither C4 nor O4 atomic basins appear in Figure 5 (top) for the experimental geometry of 2a. Integration of the electron density inside each atomic basin rendered the corresponding atomic electric charges. Table 1 Table 1. QTAIM Atomic Charges, Q(A) (e), for Selected Atoms of Complexes 2a and 3ba atomb 2a 3b

Mn1

N1

N3

C4

O4

1.153 1.116 1.100 1.075

−1.169 −1.163 −1.202 −1.237

−1.170 −1.122 −1.138 −1.139

1.012 0.986 1.016 1.025

−1.193 −1.194 −1.208 −1.246

a

Experimental geometry (first row) and theoretically optimized geometry (second row). bMore values are given in the Supporting Information.

(and Figure S1) compares QTAIM charges of selected atoms for the four structures showing rather small differences between them. As expected, Mn1 atom has nearly identical positive charge (+1.1 e) in 2a and 3b, using either experimental or theoretical geometry. All N atoms attached to the Mn1 atom are negatively charged, with values ranging from −1.1 to −1.2 e. NBO charges (not included in the tables) are consistent with the above results. As shown in Figure 6, where the Coulomb electrostatic potential (ESP) is represented for the experimental geometry of both compounds, it is this magnitude (which includes multipolar expansion terms) which is much more informative in this case than the monopolar charges alone. While for the 2a complex the most negative value of ESP belongs to the N3 atom, for the 3b complex, on the contrary, it is at the O4 atom where an electrophilic attack is suggested to be more favored. Additionally, from Figure 6 (top) we may infer that N3−C4 and N3− C20 interactions in complex 2a are mainly of noncovalent electrostatic (i.e., ionic) type (see below for further discussion of this important point). There are several local (i.e., calculated at a bcp) and integral (i.e., calculated over a whole atomic basin, over an interatomic surface, or along a bond path) topological properties of the electron density that have been successfully used to analyze the bonding in compounds containing transition metals starting from both theoretical and experimental electron densities.17 Among the former, the electron density (ρb), the ellipticity (εb), the Laplacian of the electron density (∇2ρb), the kinetic energy density ratio (Gb/ρb), and the total energy density ratio (Hb/ρb, with Hb(r) = Gb(r) + Vb(r) and 1/4∇2ρb(r) = 2Gb(r) + Vb(r), where Vb(r) is the potential energy density) are by far the most common. However, the delocalization index δ(A−B), which is an integral property which can be obtained only from the theoretical electron densities, is a useful tool directly related to the number of electron pairs delocalized between atoms A and B and can be considered a covalent bond order measure.18 Values of these topological properties, along with the bond path length, for selected bonds of complexes 2a and

Figure 6. Electrostatic potential mapped on a 0.07 e Å−3 electron density isosurface for 2a (experimental geometry) (top) and 3b (experimental geometry) (bottom).

3b are collected in Table 2. Additional values of the delocalization index for more bonds are provided in Figure S2. Most interestingly, topological properties found for the N3− C4 bond in 2a complex (experimental geometry) are very different to those of a typical covalent bond between nonmetal atoms, just like the ones shown by the same bond in complexes 2a (theoretical geometry) and 3b (both geometries), as well as those exhibited by C2−N3 and N1−C2 bonds in the four compounds (see Table 2). Moreover, the N3−C4 bond in 2a complex (experimental geometry) shows properties basically identical to the ones shown by the N3−C20 bond in the same complex, in particular large bond path lengths, which are approximately twice as long as the ones in typical covalent bonds within the same molecule and also larger than internuclear distances, which are 2.940 and 2.958 Å, respectively, values which are a consequence of the curvatures observed previously in Figure 4a for the bp’s. The small,