Synthesis and Crystal Structures of P-Iron-Substituted

Article pubs.acs.org/Organometallics

Synthesis and Crystal Structures of P-Iron-Substituted Phosphinoborane Monomers Kazuyuki Kubo,* Tomohiro Kawanaka, Masao Tomioka, and Tsutomu Mizuta* Department of Chemistry, Graduate School of Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima, Hiroshima 739-8526, Japan S Supporting Information *

ABSTRACT: A family of P-iron-substituted phosphinoboranes, Cp(CO)2Fe{P(Ar)BMes2} (Ar = Ph, Mes, Tipp, Mes*), have been prepared from the reaction of Cp(CO)2FeCl and (Li)(Ar)PBMes2. All the complexes have been characterized successfully by 1H, 11B, and 31P NMR; IR spectroscopy; and Xray crystallography. In the IR spectra, all the complexes display similar carbonyl stretching frequencies that are remarkably higher than those of closely related phosphide complexes. These observations indicate that a repulsive interaction between the filled d orbital on the iron and the lone pair on the phosphorus is less severe in the studied iron-phosphinoboranes, which is most likely because of the P→B π interaction that occurs in them. The 31P{1H} NMR chemical shifts of the phosphinoborane phosphorus move upfield with the increasing steric bulk of the Ar groups in the order Ph (−51.4 ppm) < Mes (−68.8 ppm) < Tipp (−84.9 ppm). However, the phosphorus bearing the most sterically demanding Mes* group appears at an unexpectedly downfield value of −44.9 ppm, which is probably reflective of its structural peculiarities. The 1H NMR spectrum of each complex displays two sets of signals, assignable to inequivalent Mes groups on the boron atom, as a consequence of a hindered rotation around the P−B bond. This high rotational barrier most likely results from the significant double-bond character in the P−B bond. The X-ray diffraction studies have confirmed the iron-phosphinoboranes considered herein to be monomeric species. Each molecule consists of a nearly planar phosphinoborane fragment with a short P−B bond. The Fe−P bond is notably elongated as the Ar group becomes larger, demonstrating its somewhat vulnerable nature with respect to steric congestion. In contrast, the variation in the P−B bond distance is relatively small throughout the series of iron-phosphinoboranes, suggesting that the PB double-bond character is balanced by steric and electronic effects of the substituents around the phosphorus.

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INTRODUCTION The chemistry of group 13/15 compounds has extensively grown over the past decades and continues to be an active research area in academic and industrial laboratories.1−6 Recent research focus in this area has mostly been application-driven, e.g., quests for novel semiconducting materials, advanced ceramics, promising inorganic polymers,1,2 and hydrogenstorage materials.3 Nevertheless, fundamental interests in the bonding nature and reactivity of simple molecules remain unabated. In particular, compounds containing a π bond between a lone pair on the group 15 element and an empty p orbital on the group 13 element are of special importance, partly because of their isoelectronic relationship to organic counterparts having a carbon−carbon π bond.1,4 Unlike the planar geometry of the nitrogen atom in aminoborane H2N BH2, the phosphorus atom in phosphinoborane H2PBH2 is significantly pyramidal. Phosphorus tends not to produce a planar geometry because of its higher inversion barrier as compared to that of nitrogen.5 This nonplanarity reduces the intramolecular π overlap, thereby increasing the basicity of the phosphorus and the acidity of the boron atom. These unique features of phosphinoboranes are responsible for their interesting electronic structure and appealing reactivity profile.6 Because of their amphoteric property, phosphinoboranes have a © 2012 American Chemical Society

strong propensity for intermolecular head-to-tail aggregation, through which they can form a variety of condensed products. In particular cases, they even undergo H−H or C−H bond activation,7 demonstrating parallel behavior to “frustrated Lewis pairs”.8 The fascinating reactivity profile of this system has attracted increasing interest in recent years.9,10 In order to further develop the chemistry of phosphinoboranes, sterically and electronically tunable substituent frameworks are highly desirable, because the properties of phosphinoboranes are strongly dependent on the substituents.6 In general, the introduction of bulky substituents onto the phosphinoborane framework is effective for increasing π bonding and avoiding aggregation. Imposed steric congestion not only hinders the intermolecular association but also encourages the phosphorus to assume planar geometry and hence enhance the π overlap. In addition, it is considered that the electropositive and electronegative substituents on the phosphorus and boron, respectively, would increase the donor−acceptor π interaction. In light of these steric and electronic considerations, transition-metal fragments would be among the most desirable Received: December 31, 2011 Published: February 14, 2012 2026

dx.doi.org/10.1021/om201287s | Organometallics 2012, 31, 2026−2034

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deeper understanding of the fundamental steric and electronic factors contributed by a transition-metal fragment that affect the nature of phosphinoboranes, we report the synthesis and characterization of P-iron-substituted phosphinoborane monomers with a substantial PB double-bond character.

substituents for phosphorus, because of their tunable steric demand and electronic nature. To our knowledge, there is only one reported example of a well-defined P-transition-metalated phosphinoborane (Chart 1, Type I) thus far,11 whereas several

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Chart 1. Metalated Phosphinoboranes

RESULTS AND DISCUSSION Synthesis and Spectroscopic Trends of Iron-Phosphinoboranes. The iron-phosphinoboranes Cp(CO)2FeP(Ar)BMes2, 1−4, were prepared according to a facile procedure (Scheme 2). All the syntheses involved the reaction of a 1:1 Scheme 2

complexes bearing a datively bonded phosphinoborane (Chart 1, Type II and III) are known.12 Paine and co-workers have synthesized a zirconium complex in which two −P(H)B(NiPr2){N(SiMe3)2} fragments are covalently bonded to the metal center.11 The X-ray crystallographic study revealed that each phosphorus atom had a pyramidal geometry with a considerably long P−B bond, while the boron and nitrogen atoms were found to be trigonal planar.11 These results clearly suggest that dominant N→B π donation competed with the P→B π donation to deplete the PB bond character. Additionally, the zirconium atom, as a π acceptor, further suppressed the P−B π interaction to some extent (Chart 2, a). mixture of Cp(CO)2FeCl and lithiated phosphinoborane (Li)(Ar)PBMes2 (59−93% yield). Selected spectroscopic data for 1−4 are summarized in Table 1. The NMR and IR data

Chart 2. Possible Interactions between a Transition-Metal d Orbital and a PB π Bonda

Table 1. IR, 31P{1H} NMR, and 11B{1H} NMR Data for 1− 4a a

Shading represents orbital occupation.

−1 b

IR (cm ) 31 1 P{ H} NMR (δ)c 11 1 B{ H} NMR (δ)c

Previously, we reported that dehydrohalogenation from a trichloroborane adduct of an iron-phosphide complex provided a P-iron-substituted phosphinoborane dimer, probably via the immediate association of transient monomers during the course of the reaction (Scheme 1).13 The results indicated that the putative metalated phosphinoborane monomer was highly amphoteric. It seemed that the chloro substituents on the boron atom were sterically less demanding, which allowed the association to proceed. Another important factor would be the repulsive interaction between an occupied dπ orbital on the late transition metal and the phosphorus lone pair (Chart 2, b). The enhanced basicity of the phosphorus lone pair likely encourages the intermolecular association. This speculation, on the other hand, leads to the possibility that the late-transition-metal fragment might also effectively promote the intramolecular P→ B π donor−acceptor interaction, provided the aggregation is inhibited by more sterically hindered substituents. To achieve a

a

1

2

3

4

2027, 1979 −51.4 72.8

2022, 1976 −68.8 66.6

2022, 1976 −84.9 66.8

2020, 1974 −44.9 63.2

At room temperature. bIn THF. cIn C6D6.

were in excellent agreement with the anticipated monomeric phosphinoborane structures, as confirmed by X-ray crystallography (see below). The complex 1, bearing a Ph group on the phosphorus, is extremely sensitive to air, and therefore, purification was achieved only by the careful extraction from a crude product using highly purified solvents under an inert gas atmosphere. Complex 2, having the Mes group on the phosphorus, is relatively stable and can be purified by either extraction or column chromatography performed quickly under air. Complexes 3 and 4 respectively have a very bulky Tipp group and Mes* group on the phosphorus, and their stabilities are sufficient for isolation by column chromatography under air to produce a high yield. The stability of the iron-

Scheme 1

2027


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structural peculiarities, as suggested by structural studies (see below). The chemical shifts of the iron-phosphinoboranes are far upfield of the chemical shift of the zirconium complex (37.3 ppm)11 mentioned above. Power and co-workers reported that lithiated phosphinoboranes resonate in the far downfield region, partly because of the highly ionic nature of the Li−P bond (e.g., (Li)(Ph)PBMes2, 73.1 ppm; (Li)(Mes)PBMes2, 55.5 ppm).19−22 These trends suggest that the Fe−P bond in the present complexes is essentially covalent and not polarized to a significant extent, unlike the Li−P bond. In addition, the signals of the iron complexes are substantially upfield of those observed in the case of the related phosphinoboranes with hydrocarbyl substituents (e.g., Ph 2 PBMes 2 , 26.7 ppm; Mes2PBMes2, 27.4 ppm),20,23 whereas they are comparable to or slightly upfield of those of hydro-19 or silyl-phosphinoboranes20 (e.g., (H)(Ph)PBMes2, −41.5 ppm; (H)(Mes)PBMes2, −66.0 ppm; (Ph3Si)(Ph)PBMes2, −35.1 ppm). The 11B chemical shift of each compound 1−4 (63.2−72.8 ppm) is in the three-coordinate boron region,6 consistent with the monomeric formulation in solution. This behavior is in stark contrast to that of the aforementioned Cp(CO)2FeP(Ph)BCl2, which promptly dimerizes, giving an upfield chemical shift at 4.6 ppm.13 Furthermore, the VT 11B{1H} NMR spectrum of the dimer showed no apparent change in its chemical shift up to 70 °C, indicating that no dimer−monomer equilibrium exists at these temperatures (Scheme 3).24 In this

phosphinoboranes could be attributed to the steric shielding by the phosphorus substituent. The relative donor strength of ligands is commonly determined by comparing the infrared stretching frequencies of terminal carbonyl ligands in the corresponding metal complexes. All the complexes exhibit similar infrared spectra, featuring a symmetric stretching band at 2020−2027 cm−1 and an asymmetric stretching band at 1974−1979 cm−1. These observations indicate that the electronic and geometric environment around the iron center is very similar for 1−4. Interestingly, in comparison with structurally related complexes (Chart 3), the iron-phosphinoboranes, namely, borylphosphide Chart 3. Phosphide Complexes and P-Iron-Substituted Phosphinoboranea

a

Shading represents orbital occupation.

complexes, have CO absorption bands at a remarkably higher frequency than phosphide complexes such as Cp(CO)2FePPh2 (2008, 1956 cm−1)14 and Cp(CO)2FePPh{N(SiMe3)2} (1985, 1931 cm−1).15 These observations point at the decreased electron density of the metal center in the iron-phosphinoboranes, despite the presence of a more electropositive boron atom on the phosphorus. In contrast, the electronic properties of the phosphinoborane ligands seem to be more comparable to those of a borane adduct of the phosphide ligand such as in Cp(CO)2FeP(Ph2)BH3 (νCO: 2031, 1983 cm−1),14,16 where the lone pair on the phosphorus is completely masked by the borane molecule. In phosphide complexes with a coordinatively saturated metal center, a repulsive interaction is commonly encountered between the filled d orbital on a metal center and the lone pair on the phosphide phosphorus.17 This interaction would enhance the d→CO π* back-donation to shift the CO absorption bands to a lower frequency. Taking into account the high νCO values for 1−4, the lone-pair repulsion appears to be less severe in the metalated phosphinoboranes. This is probably because the electron density at the phosphorus atom is considerably decreased by the intramolecular P→B π donation. The 31P{1H} NMR spectrum of 1−4 shows a signal over a somewhat wide range varying from −84.9 to −44.9 ppm, despite the similar donor strength of the phosphorus, as suggested by the CO stretching frequencies. Unfortunately, coupling constants between the phosphorus and boron atoms could not be determined because of the inherent broadness of the peaks resulting from the quadrupolar nature of the boron nucleus. The chemical shifts of 1−3 move upfield in the order Ar = Ph (−51.4 ppm) < Mes (−68.8 ppm) < Tipp (−84.9 ppm). Considering the similar Hammett σ-constants for Me and iPr groups,18 this observation is likely associated with the increased steric bulk, rather than inductive effects (+I). However, the phosphorus in 4, which bears the most sterically demanding Mes* group, appears at an unexpectedly downfield value of −44.9 ppm. This anomaly might be attributable to its

Scheme 3. Dimer−Monomer Equilibrium

manner, the protection on the boron atom by bulky (and also less electronegative) substituents is shown to be crucial to avoid the association of the monomers. Complete signal assignment in the 13C{1H} NMR spectrum of 1−4 was made difficult by signal overlapping, coupling with the phosphorus nucleus, broadening due to the boron nucleus, and inherently low intensities of some occurrences of resonance, especially in the aromatic carbon region. Fortunately, the signal assignable to the terminal carbonyls is well resolved and appears as a doublet in each complex owing to the coupling with the phosphinoborane phosphorus atom (1, 212.6 ppm, JPC = 19 Hz; 2, 213.7 ppm, JPC = 21 Hz; 3, 213.6 ppm, JPC = 18 Hz; 4, 215.1 ppm, JPC = 20 Hz). These coupling constants are greater than that observed for the dimer [Cp(CO)2FeP(Ph)BCl2]2 (JCP = 10 Hz).13 The larger couplings obtained in the monomers can be reasonably accounted for by the sp2hybridization of the phosphorus in the monomers, which would cause more s-orbital character in the bonding than the sp3hybridized phosphorus in the dimer. 1 H NMR spectral data provide further information on the nature of the P−B bonding in phosphinoboranes.20 All the iron-phosphinoboranes exhibit a characteristic NMR pattern of two sets of two singlets assignable to o- and p-Me groups in the Mes groups on the boron atom. This indicates that the rotation around the P−B bond is restricted to be frozen or at least slow on the NMR time scale, and therefore the boron Mes groups 2028


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are inequivalent. The VT 1H NMR spectra of 1−4 revealed that this signal pattern remained fundamentally unchanged at elevated temperatures up to 70 °C. It can be deduced that the P−B bond in iron-phosphinoboranes has a significant double-bond character, demonstrating its high rotation barrier. It is also notable that one of the two o-Me signals due to the Mes groups in 3 and 4 is significantly broadened at room temperature but sharpens at elevated temperatures. In order for the two o-Me groups in a Mes group to be equivalent, a certain rotational process would be needed for the B−Mes bond on the NMR time scale,25 considering the propeller-like screwed conformation of the Mes groups (see below). Therefore, the observations of the broad Mes peaks in 3 and 4 may indicate that the rotation around the B−Mes bond is also restricted in the crowded phosphinoboranes. Structural Characterization of Iron-Phosphinoboranes. To unambiguously characterize these complexes and to get a direct insight into the PB bonding nature, X-ray crystallographic studies were performed on all the complexes described herein. The ORTEP diagrams are given in Figures 1−4, along with selected structural parameters. The solid-state Figure 2. Molecular structure and atom-labeling scheme for one of the two crystallographically independent molecules of 2. Hydrogen atoms and the lattice solvent are omitted for clarity. Thermal ellipsoids represent 50% probability. Selected bond lengths (Å) and angles (deg): (a) Fe(1)−P(1) 2.2986(5), P(1)−B(1) 1.833(2), P(1)−C(24) 1.8305(18), C(6)−B(1) 1.592(3), C(15)−B(1) 1.596(3), C(24)− P(1)−Fe(1) 114.05(6), B(1)−P(1)−Fe(1) 128.34(7), C(24)−P(1)− B(1) 115.41(9), C(6)−B(1)−P(1) 117.15(13), C(15)−B(1)−P(1) 121.58(13), C(6)−B(1)−C(15) 121.09(15). (b) Fe(2)−P(2) 2.3156(6), P(2)−B(2) 1.836(2), P(2)−C(58) 1.8338(18), C(40)− B(2) 1.590(3), C(49)−B(2) 1.597(3), C(58)−P(2)−Fe(2) 113.78(6), B(2)−P(2)−Fe(2) 129.60(7), C(58)−P(2)−B(2) 114.09(9), C(40)−B(2)−P(2) 116.28(14), C(49)−B(2)−P(2) 121.36(14), C(40)−B(2)−C(49) 122.06(15). Atoms C58, C40, and C49 in molecule b correspond to C24, C6, and C15 in molecule a, respectively.

orbital interaction between the phosphorus and boron atoms rather than from steric demands, which would lead to an orthogonal or staggered orientation between the phosphorus and boron planes around the P−B bond. In all the complexes, the largest angle around the phosphorus atom is the Fe−P−B angle, and hence, the repulsion between the iron fragment and the boron fragment is presumably the most severe. This suggests that the steric demand of the iron fragment is greater than that of the planar Ar groups on the phosphorus. The Mes groups on the boron and the aromatic group on the phosphorus exhibit a propeller-like twist conformation that places the ortho substituents out of the phosphinoborane plane to minimize the overall steric repulsion. Apparently, those substituents located above and below the phosphinoborane plane lead to the great stabilization of the molecule by steric shielding. One of the two carbonyl ligands on the iron atom lies in the phosphinoborane plane, whereas the other is nearly perpendicular to the plane. The in-plane carbonyl is oriented on the same side of the Mes group on the boron in 1, whereas those in 2−4 are on the side of the arene ring on the phosphorus. These orientations of the metal fragment seem to be favorable to minimize not only the steric repulsion between the substituents but also the electronic repulsion between the filled d orbital on the iron center and the lone pair on the phosphorus.17,26

Figure 1. Molecular structure and atom-labeling scheme for 1. Hydrogen atoms are omitted for clarity. Thermal ellipsoids represent 50% probability. Selected bond lengths (Å) and angles (deg): Fe(1)− P(1) 2.2810(8), P(1)−B(1) 1.838(3), P(1)−C(24) 1.826(3), C(6)− B(1) 1.603(4), C(15)−B(1) 1.593(4), C(24)−P(1)−Fe(1) 113.68(9), B(1)−P(1)−Fe(1) 126.79(11), C(24)−P(1)−B(1) 109.94(13), C(6)−B(1)−P(1) 120.5(2), C(15)−B(1)−P(1) 119.7(2), C(15)−B(1)−C(6) 119.5(2).

structure of 2 shows the presence of two crystallographically independent molecules of 2 in the asymmetric unit. They are very similar in structure, and one of the molecules is illustrated in Figure 2. It is clear that the monomeric structures of complexes 1−4 agree with the spectroscopic data of their solutions. Overall, each molecule consists of a normal three-legged piano-stool iron fragment and a nearly planar phosphinoborane fragment. The sum of the angles around the phosphorus varies from 350° to 360°, whereas the boron is almost planar (359°−360°). This ethylene-like geometry most likely results from the strong π2029


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phosphorus: 346.6(2)° and 341.3(2)°) with long P−B bond lengths (1.916(7) and 1.926(6) Å).11 These data clearly indicate that the P−B bonds in this complex should be regarded as single bonds. In contrast, for the complexes 1−4, the P−B bonds (1.833(2)−1.857(2) Å) are significantly shorter than the criterion value and comparable to those of the closely related organophosphinoboranes previously reported.6 These results support the conclusion that the P−B bonds in iron phosphinoboranes have a high degree of double-bond character. To obtain a deeper understanding of the influence of a transition-metal fragment on the P−B π interactions, a close inspection and a direct comparison of their structural parameters would be very useful (Table 2). Detailed characterizations were conducted, focusing on the geometry of the phosphorus, by considering the key angular parameters, namely, the angles ϕ and ω and the bond distances.27 The definitions of these angles are presented in Chart 4 through simplified models. In brief, the angle ϕ is a measure of pyramidalization of the phosphorus geometry, and it is defined by the angle between the phosphorus plane and the molecular plane, which includes the P−B bond. The angle ω is the rotation angle between the phosphorus plane and the boron plane when viewed along the P−B bond. The exact definitions of these angles are presented in the Supporting Information. These parameters roughly illustrate the degree of a slant of the phosphorus lone pair with respect to the boron p orbital, thus offering a reasonable estimation of the distortion of the π system. Basically, the relatively small differences in their metrical parameters are likely to arise from mere crystal-packing effects. However, several interesting points can be noted from the systematic trends in Figure 5. As can be seen in Figure 5a−c, the bond distances around the phosphorus atom in 1−4 tend to increase with the increasing steric bulk of the substituent on the phosphorus. The elongation is also most remarkable in the Fe− P bonds and less profound in the P−B bonds. This observation suggests that the iron−phosphorus bond is relatively vulnerable and easily elongated to relieve the steric crowding around the phosphorus. Among the complexes, 4 has a distinctively long Fe−P bond, indicating the extremely severe repulsion in 4. The smaller ϕ angle and larger ω angle of 4 are also consistent with this conjecture (Figure 5d and e). The Mes* group is surprisingly bent (Figure 4), likely because of the intramolecular steric repulsion that is often observed in compounds bearing a Mes* group. This unique feature of 4 could be partly responsible for its unexpected downfield shift in the 31P NMR spectrum and the broadening of the o-Me signal in the 1H NMR spectrum. Experimental20 and theoretical5 studies have shown the close relationship between the degrees of phosphorus pyramidalization (i.e., ϕ angle) and the PB double-bond length (see Introduction). Sterically less encumbered phosphinoboranes have a relatively larger ϕ angle. That is, they have a pyramidal geometry, leading to a longer P−B bond than molecules that are sterically more hindered and have a smaller ϕ angle. Power and co-workers have calculated the structures of phosphinoboranes H2PBH2 and (H3C)2PB(CH3)2 by varying the ϕ angle, using the B3LYP density functional method.5c The optimized geometries displayed systematic decrease in the PB bond length from 1.9295 Å to 1.8017 Å for H2PBH2 and from 2.0021 Å to 1.8176 Å for (H3C)2PB(CH3)2 when the ϕ angle was decreased from 90° to 0° (planar). In agreement with the

Figure 3. Molecular structure and atom-labeling scheme for 3. Hydrogen atoms and the lattice solvent are omitted for clarity. Thermal ellipsoids represent 50% probability. Selected bond lengths (Å) and angles (deg): Fe(1)−P(1) 2.3153(6), P(1)−B(1) 1.836(2), P(1)−C(24) 1.8459(19), C(6)−B(1) 1.609(3), C(15)−B(1) 1.601(3), C(24)−P(1)−Fe(1) 110.15(6), B(1)−P(1)−Fe(1) 127.90(7), B(1)−P(1)−C(24) 118.76(9), C(6)−B(1)−P(1) 115.70(14), C(15)−B(1)−P(1) 122.10(14), C(15)−B(1)−C(6) 121.57(16).

Figure 4. Molecular structure and atom-labeling scheme for 4. Hydrogen atoms are omitted for clarity. Thermal ellipsoids represent 50% probability. Selected bond lengths (Å) and angles (deg): Fe(1)− P(1) 2.3740(5), P(1)−B(1) 1.857(2), P(1)−C(24) 1.8746(18), C(6)−B(1) 1.611(3), C(15)−B(1) 1.615(3), C(24)−P(1)−Fe(1) 122.91(6), B(1)−P(1)−Fe(1) 124.64(7), B(1)−P(1)−C(24) 112.34(9), C(6)−B(1)−P(1) 116.38(13), C(15)−B(1)−P(1) 124.00(14), C(6)−B(1)−C(15) 119.57(15).

It was proposed that a P−B bond less than 1.9 Å long has some π-bonding character.20 In the above-mentioned zirconium complex Cp2Zr{P(H)B(NiPr2)[N(SiMe3)2]}2, each phosphorus atom assumes a pyramidal geometry (the angles around 2030


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Table 2. Selected Structural Parameters for 1−4 Fe−P distance (Å) P−C distance (Å) P−B distance (Å) angle ϕ (deg) angle ω (deg) a

1

2aa

2ba

3

4

2.2810(8) 1.826(3) 1.838(3) 29.6 1.1

2.2986(5) 1.8305(18) 1.833(2) 14.0 1.3

2.3156(6) 1.8338(18) 1.836(2) 14.9 1.5

2.3153(6) 1.8459(19) 1.836(2) 16.3 9.7

2.3740(5) 1.8746(18) 1.857(2) 3.6 12.3

Crystallographically independent molecules.

Chart 4. Angular Parameters ϕ and ω

overly severe steric repulsion around the phosphorus, as discussed above.

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CONCLUSION We synthesized a family of iron-phosphinoboranes that have a significant PB double-bond character. The NMR and IR data are consistent with observations on their monomeric structures in solution. The NMR studies revealed that the P−B bond in the iron-phosphinoborane has a high rotational barrier, indicating its significant double-bond character. As a result of the P−B donor−acceptor π interaction, the repulsive dπ(Fe)− pπ(P) interaction commonly encountered in phosphide complexes appears to be less severe in the metalated phosphinoborane system. Structural characterization by X-ray diffraction revealed that the phosphinoborane fragment possesses a planar geometry with a short P−B bond, corroborating its significant double-bond character. Notably, among the metric parameters around the phosphorus, the Fe− P bond distance is significantly responsive to variations in the surrounding steric environment, indicating that the Fe−P bond is relatively vulnerable by nature. In contrast, the variation in the P−B bond distance is less substantial, which suggests that the PB double-bond character is balanced by the steric and electronic effects of the substituents. The present study provides valuable insights into the nature of the metalated phosphinoboranes. The compounds reported here represent a new class of organometallic compounds that have a potentially amphoteric property and can serve as multifunctional molecules.28 The interaction between a transition-metal fragment and a phosphinoborane fragment will be presented

computational results, Ph2PBPh2 with a larger ϕ angle (42.1°) had a longer P−B bond of 1.859(3) Å than the more congested Mes2PBMes2, which had an almost planar geometry (ϕ = 2.2°) and a P−B bond of 1.839(8) Å.20 Thus, one might expect that 1, with a larger ϕ angle (29.6°), should have a longer P−B bond. However, this is apparently not the case in the present system, where the P−B bond in 1 (1.838(3) Å) is comparable to those in the other complexes. Presumably, the P−B bond distance is balanced by the steric and electronic effects of the substituents as follows. The π-donating iron fragment likely enhances the availability of the phosphorus lone pair, encouraging the P−B π interaction. Although an uncrowded complex has a large ϕ angle, a short Fe−P bond leads to strong electron donation from the iron to the phosphorus, thereby compensating for the disadvantage of the pyramidalization and resulting in the contraction of the P−B bond. Moreover, decreased steric repulsion leads to a smaller ω angle, which provides a PB bond with better pπ−pπ overlap. Although a complex bearing a large Ar group on the phosphorus has a smaller ϕ angle, favoring the π interaction, its longer Fe−P bond and larger ω angle would result in the weakening of the PB bond. As a consequence, complexes 1−3 exhibit a similar P−B distance, even though their parameters are distinctly different. Only 4 has a relatively long P−B bond because of the

Figure 5. Comparisons between key structural parameters of iron-phosphinoboranes. (a) Fe−P bond distances (Å). (b) P−C bond distances (Å). (c) P−B bond distances (Å). (d) ϕ angles (deg). (e) ω angles (deg). Data for 2 are average values of two crystallographically independent molecules 2a and 2b. 2031


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Table 3. Crystallographic Data for 1−4 empirical formula fw T (K) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalcd (g cm−3) λ (Å) μ (mm−1) θ range (deg) R1, wR2 (I > 2σ(I))a a

1

2

3

4

C31H32BFeO2P 534.20 173 monoclinic P21/n 10.2719(17) 18.659(3) 14.278(2) 90 96.798(2) 90 2717.3(8) 4 1.306 0.71073 0.640 1.80 to 27.48 0.0464, 0.1010

C35.5H41.5BFeO2P 597.82 173 triclinic P1̅ 11.6059(13) 12.6911(15) 21.790(3) 86.1750(10) 80.8380(10) 86.113(2) 3156.1(6) 4 1.258 0.71073 0.558 1.61 to 27.50 0.0369, 0.0887

C43H57BFeO2P 703.52 200 monoclinic P21/c 10.44000(10) 15.39400(10) 25.1120(3) 90 99.1310(10) 90 3984.69(7) 4 1.173 0.71069 0.452 3.30 to 27.50 0.0475, 0.1071

C43H56BFeO2P 702.51 200 monoclinic P21/c 11.4640(2) 18.9210(3) 17.6940(4) 90 90.6480(10) 90 3837.77(13) 4 1.216 0.71069 0.469 2.15 to 27.50 0.0438, 0.1032

R1 = ∑∥Fo| − |Fc∥/∑|Fo|, wR2 = {∑[w(Fo2 − Fc2)2]/∑[(wFo2)2]}1/2, w = 1/[σ2(Fo2) + (aP)2 + bP], P = (Max(Fo2, 0) + 2Fc2)/3. Synthesis of 2. This compound was prepared from Cp(CO)2FeCl (608 mg, 2.86 mmol) and (Li)(Mes)PBMes2 (2.55 mmol) using a similar procedure to that described for 1, yielding a purple powder (1.411 mg, 2.36 mmol, 93% yield). Crystals of 2 suitable for X-ray analysis were formed from a hot hexane solution (60 °C) that was slowly cooled to room temperature and allowed to stand for a few days. Anal. Calcd for C34H38BFeO2P·1/4C6H14: C, 71.32; H, 7.00. Found: C, 71.10; H, 7.04. IR (νCO, in THF): 2022, 1976 cm−1. 1H NMR (δ, in C6D6): 2.02 (s, 3H, Mes p-CH3), 2.06 (s, 3H, Mes pCH3), 2.27 (s, 3H, Mes p-CH3), 2.64 (s, 6H, Mes o-CH3), 2.74 (s, 6H, Mes o-CH3), 2.82 (s, 6H, Mes o-CH3), 4.05 (s, 5H, C5H5), 6.74 (s, 2H, Mes m-H), 6.78 (s, 2H, Mes m-H), 6.89 (s, 2H, Mes m-H). 31 1 P{ H} NMR (δ, in C6D6): −68.8 (br). 11B{1H} NMR (δ, in C6D6): 66.6 (br). Synthesis of 3. An ether solution (ca. 40 mL) of (Li)(Tipp)PBMes2 (3.90 mmol) was added dropwise under stirring to an ether solution (40 mL) of Cp(CO)2FeCl (657 mg, 3.09 mmol) at −78 °C. The mixture was allowed to warm to room temperature and stirred for 1.5 h. The resulting solution was filtered, and the filtrate was concentrated to dryness. The resulting oily residue was transferred to an alumina column with hexane and eluted with hexane, after which it was eluted with a 1:9 toluene/hexane mixture. A broad purple band was collected and reduced to dryness to yield a purple powder (1.613 mg, 2.29 mmol, 74% yield). Crystals of 3 suitable for X-ray analysis were formed from a hot hexane solution (60 °C) that was slowly cooled to −30 °C and allowed to stand for a few days. Anal. Calcd for C40H50BFeO2P·1/2C6H14: C, 73.41; H, 8.17. Found: C, 72.78; H, 7.62. IR (νCO, in THF): 2022, 1976 cm−1. 1H NMR (δ, in C6D6): 1.19 (d, JHH = 6.6 Hz, 6H, Tipp CH3), 1.27 (br, 6H, Tipp CH3), 1.43 (d, JHH = 6.9 Hz, 6H, Tipp CH3), 2.09 (s, 3H, Mes p-CH3), 2.28 (s, 3H, Mes p-CH3), 2.63 (s, 6H, Mes o-CH3), 2.81 (br, 7H, Tipp p-CHMe2 and Mes o-CH3), 4.08 (s, 5H, C5H5), 4.41 (br, 2H, Tipp o-CHMe2), 6.71 (s, 2H, Mes or Tipp m-H), 6.87 (s, 2H, Mes or Tipp m-H), 7.10 (s, 2H, Mes or Tipp m-H). 31P{1H} NMR (δ, in C6D6): −84.9 (br). 11 1 B{ H} NMR (δ, in C6D6): 66.8 (br). Synthesis of 4. This compound was prepared from Cp(CO)2FeCl (305 mg, 1.44 mmol) and (Li)(Mes*)PBMes2 (0.90 mmol) using a similar procedure to that described for 3, using a silica gel column for purification instead of an alumina column, to yield a green powder (523 mg, 0.74 mmol, 83% yield). Crystals of 4 suitable for X-ray analysis were formed from a hot 3:2 mixture of THF and MeCN (50 °C) that was slowly cooled to room temperature and allowed to stand for a few days. Anal. Calcd for C43H56BFeO2P: C, 73.51; H, 8.03. Found: C, 73.51; H, 7.83. IR (νCO, in THF): 2020, 1974 cm−1. 1H

in a future work in further detail with the help of DFT calculations.

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EXPERIMENTAL SECTION

General Information. All reactions were carried out using standard Schlenk techniques under an atmosphere of dry nitrogen, unless otherwise stated. Solvents were distilled from appropriate drying agents or purified by a Glass Contour solvent purification system and kept under an atmosphere of dry nitrogen. Flash column chromatography was carried out quickly under an ambient atmosphere using silica gel 60 (0.040−0.063 mm) (Merck) or aluminum oxide 90 standardized (Merck). All other commercial reagents were used without further purification. Cp(CO)2FeCl29 was prepared as described in the literature. (Li)(Ar)PBMes2 (Ar = Ph, Mes, Tipp, Mes*) were synthesized according to a literature method19−22 or modifications thereof and used without isolation for subsequent reactions. NMR spectra were collected on a 300 MHz JEOL Lambda spectrometer. 1H NMR chemical shifts are reported relative to SiMe4 and were determined by reference to the residual 1H solvent peak. 31 1 P{ H} and 11B{1H} NMR chemical shifts are reported relative to the external references H3PO4 (85%) and Et2OBF3, respectively. IR measurements were recorded on a Perkin-Elmer Spectrum One FT-IR spectrometer. Elemental analyses were performed on a PerkinElmer 2400 Series II CHNS/O elemental analyzer. Synthesis of 1. An ether solution (ca. 40 mL) of (Li)(Ph)PBMes2 (2.49 mmol) was added dropwise under stirring to a THF solution (40 mL) of Cp(CO)2FeCl (524 mg, 2.47 mmol) at −78 °C. The mixture was allowed to warm to room temperature and stirred overnight. The resulting black solution was then concentrated to dryness, and the resulting oil was washed with hexane (3 × 10 mL). The supernatant solution was decanted, and the purple solid was dried in vacuo. After extraction with toluene (20 mL, and then, 5 mL), the volatile components were removed from the combined extract. The resulting solid was washed with hexane (5 mL) and dried in vacuo to afford a purple powder (780 mg, 1.46 mmol, 59% yield). Crystals of 1 suitable for X-ray analysis were formed from a hot hexane solution (60 °C) that was slowly cooled to −30 °C and allowed to stand for a few days. Anal. Calcd for C31H32BFeO2P: C, 69.70; H, 6.04. Found: C, 68.70; H, 6.29. IR (νCO, in THF): 2027, 1979 cm−1. 1H NMR (δ, in C6D6): 2.19 (s, 3H, Mes p-CH3), 2.30 (s, 3H, Mes p-CH3), 2.56 (s, 6H, Mes oCH3), 2.74 (s, 6H, Mes o-CH3), 4.05 (s, 5H, C5H5), 6.76 (s, 2H, Mes m-H), 6.69 (s, 2H, Mes m-H), 7.00−7.53 (m, 5H, C6H5). 31P{1H} NMR (δ, in C6D6): −51.4 (br). 11B{1H} NMR (δ, in C6D6): 72.8 (br). 2032


Organometallics

Article

NMR (δ, in C6D6): 1.39 (s, 9H, Mes* p-CCH3), 1.80 (s, 18H, Mes* oCCH3), 2.16 (s, 3H, Mes p-CH3), 2.26 (s, 3H, Mes p-CH3), 2.43 (s, 6H, Mes o-CH3), 2.77 (br, 6H, Mes o-CH3), 4.17 (s, 5H, C5H5), 6.64 (s, 2H, Mes or Mes* m-H), 6.84 (s, 2H, Mes or Mes* m-H), 7.42 (s, 2H, Mes or Mes* m-H). 31P{1H} NMR (δ, in C6D6): −44.9 (br). 11 1 B{ H} NMR (δ, in C6D6): 63.2 (br). X-ray Measurements and Structure Determination. Each crystal coated with inert oil was mounted on glass fiber and fixed in a cold nitrogen gas stream. Intensity data for 1 and 2 were measured on a Bruker APEX-II Ultra CCD-based diffractometer at 173 K, using the Bruker Apex2 software.30 Intensity data for 3 and 4 were collected on a Mac Science DIP2030 imaging plate diffractometer at 200 K and then processed using the HKL program package.31 The structures were determined by direct methods, and then, they were refined by fullmatrix least-squares methods on F2 using the SHELX-97 program package.32 ORTEP drawings were made using the ORTEP III program.33 Hydrogen atoms of 4 and those of the solvent molecule cocrystallized with 3 were placed in ideal positions and refined as riding atoms. All other hydrogen atoms were determined from subsequent difference Fourier maps and allowed positional refinement. The non-hydrogen atoms were refined anisotropically, while hydrogen atoms were refined isotropically. Crystal data, data collection parameters, and results of the analyses are summarized in Table 3. Further details are included in the Supporting Information.

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10288−10289. (b) Sutton, A. D.; Burrell, A. K.; Dixon, D. A.; Garner, E. B. III; Gordon, J. C.; Nakagawa, T.; Ott, K. C.; Robinson, J. P.; Vasiliu, M. Science 2011, 331, 1426−1429. (c) Hügle, T.; Hartl, M.; Lentz, D. Chem.−Eur. J. 2011, 17, 10184−10207. (d) Chua, Y. S.; Chen, P.; Wu, G.; Xiong, Z. Chem. Commun. 2011, 47, 5116−5129. (e) Jain, I. P.; Jain, P.; Jain, A. J. Alloys Compd. 2010, 503, 303−339. (f) Staubitz, A.; Robertson, A. P. M.; Manners, I. Chem. Rev. 2010, 110, 4079−4124. (g) Smythe, N. C.; Gordon, J. C. Eur. J. Inorg. Chem. 2010, 509−521. (h) Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I. Chem. Soc. Rev. 2009, 38, 279−293. (4) Paetzold, P. Adv. Inorg. Chem. 1987, 31, 123−170. (5) For selected examples of theoretical studies on phosphinoboranes and related compounds, see: (a) Grant, D. J.; Dixon, D. A. J. Phys. Chem. A 2005, 109, 10138−10147. (b) Grant, D. J.; Dixon, D. A. J. Phys. Chem. A 2006, 110, 12955−12962. (c) Ditty, M. J. T.; Power, W. P. Can. J. Chem. 1999, 77, 1951−1961. (d) Jemmis, E. D.; Subramanian, G. J. Phys. Chem. 1994, 98, 8937−8939. (e) Allen, T. L.; Fink, W. H. Inorg. Chem. 1992, 31, 1703−1705. (f) Coolidge, M. B.; Borden, W. T. J. Am. Chem. Soc. 1990, 112, 1704−1706. (g) Allen, T. L.; Scheiner, A. C.; Schaefer, H. F. III. Inorg. Chem. 1990, 29, 1930−1936. (h) Magnusson, E. Aust. J. Chem. 1986, 39, 735−745. (i) Magnusson, E. Tetrahedron 1985, 41, 5235−5240. (j) Gropen, O. J. Mol. Struct. 1977, 36, 111−120. (6) For selected reviews on phosphinoboranes and related compounds, see: (a) Fischer, R. C.; Power, P. P. Chem. Rev. 2010, 110, 3877−3923. (b) Power, P. P. Chem. Rev. 1999, 99, 3463−3503. (c) Driess, M.; Grützmacher, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 828−856. (d) Paine, R. T.; Nöth, H. Chem. Rev. 1995, 95, 343−379. (e) Power, P. P.; Moezzi, A.; Pestana, D. C.; Petrie, M. A.; Shoner, S. C.; Waggoner, K. M. Pure Appl. Chem. 1991, 63, 859−866. (f) Power, P. P. Angew. Chem., Int. Ed. Engl. 1990, 29, 449−460. (7) See for example: (a) Geier, S. J.; Gilbert, T. M.; Stephan, D. W. Inorg. Chem. 2011, 50, 336−344. (b) Nyhlén, J.; Privalov, T. Eur. J. Inorg. Chem. 2009, 2759−2764. (c) Geier, S. J.; Gilbert, T. M.; Stephan, D. W. J. Am. Chem. Soc. 2008, 130, 12632−12633. (d) Karsch, H. H.; Hanika, G.; Huber, B.; Meindl, K.; König, S.; Krüger, C.; Müller, G. J. Chem. Soc., Chem. Commun. 1989, 373−375. (8) For selected recent reviews on frustrated Lewis Pairs, see: (a) Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2010, 49, 46−76. (b) Stephan, D. W. Chem. Commun. 2010, 46, 8526−8533. (c) Stephan, D. W. Dalton Trans. 2009, 3129−3136. (d) Kenward, A. L.; Piers, W. E. Angew. Chem., Int. Ed. 2008, 47, 38−41. (e) Stephan, D. W. Org. Biomol. Chem. 2008, 6, 1535−1539. (f) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124− 1126. (9) Privalov, T. Chem.−Eur. J. 2009, 15, 1825−1829. (10) Gilbert, T. M.; Bachrach, S. M. Organometallics 2007, 26, 2672− 2678. (11) Chen, T.; Duesler, E. N.; Nöth, H.; Paine, R. T. J. Organomet. Chem. 2000, 614−615, 99−106. (12) (a) Schwan, K.-C.; Vogel, U.; Adolf, A.; Zabel, M.; Scheer, M. J. Organomet. Chem. 2009, 694, 1189−1194. (b) Schwan, K.-C.; Adolf, A.; Bodensteiner, M.; Zabel, M.; Scheer, M. Z. Anorg. Allg. Chem. 2008, 634, 1383−1387. (c) Schwan, K.-C.; Adolf, A.; Thoms, C.; Zabel, M.; Timoshkin, A. Y.; Scheer, M. Dalton Trans. 2008, 5054−5058. (d) Vogel, U.; Timoshkin, A. Y.; Schwan, K.-C.; Bodensteiner, M.; Scheer, M. J. Organomet. Chem. 2006, 691, 4556−4564. (e) Schwan, K.-C.; Timoskin, A. Y.; Zabel, M.; Scheer, M. Chem.−Eur. J. 2006, 12, 4900−4908. (f) Vogel, U.; Schwan, K.-C.; Hoemensch, P.; Scheer, M. Eur. J. Inorg. Chem. 2005, 1453−1458. (g) Vogel, U.; Hoemensch, P.; Schwan, K.-C.; Timoshkin, A. Y.; Scheer, M. Chem.−Eur. J. 2003, 9, 515−519. (h) Chen, T.; Jackson, J.; Jasper, S. A.; Duesler, E. N.; Nöth, H.; Paine, R. T. J. Organomet. Chem. 1999, 582, 25−31. (i) Dou, D.; Westerhausen, M.; Wood, G. L.; Linti, G.; Duesler, E. N.; Nöth, H.; Paine, R. T. Chem. Ber. 1993, 126, 379−997. (j) Linti, G.; Nöth, H.; Paine, R. T. Chem. Ber. 1993, 126, 875−887. (13) Kubo, K.; Kanemitsu, I.; Murakami, E.; Mizuta, T.; Nakazawa, H.; Miyoshi, K. J. Organomet. Chem. 2004, 689, 2425−2428.

ASSOCIATED CONTENT

S Supporting Information *

Figures giving the definitions of the angles ϕ and ω, and a CIF file giving crystallographic data for 1−4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grant-in-Aid for Scientific Research Nos. 19550066, 22550061, 23105533, and 23550079 from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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REFERENCES

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Synthesis and Crystal Structures of P-Iron-Substituted

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