Bis(borylene) Complexes of Cobalt, Rhodium, and Iridium - American

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Bis(borylene) Complexes of Cobalt, Rhodium, and Iridium [(η5-C5H5) M(BNX2)2] (X = Me, SiH3, SiMe3): A Bonding Analysis Krishna K. Pandey* School of Chemical Sciences, Devi Ahilya University Indore, Indore 452001, India S Supporting Information *

ABSTRACT: Geometry, electronic structure, and bonding analysis of the terminal neutral bis(borylene) complexes of cobalt, rhodium, and iridium [(η 5-C5H5)M(BNX2)2] (M = Co, Rh, Ir; X = Me, SiH3, SiMe3) were investigated at the DFT/BP86/TZ2P/ ZORA level of theory. The calculated geometry of iridium complex [(η 5-C5H5)Ir{BN(SiMe3)2}2] is in excellent agreement with structurally characterized iridium complex [(η 5-C5Me5)Ir{BN(SiMe3)2}2]. Pauling, Mayer, and Nalewajski-Mrozek bond multiplicities of the optimized structures of bis(borylene) complexes show that the M−B bonds in these complexes are nearly MB double bonds. On substitution of the BNX 2 ligand by the more π-acidic CO ligand, the calculated M−B bond distances increase, while substitution of the BNX 2 ligand by the less πacidic PMe3 ligand results in a decrease of the calculated M−B bond distances. The acute B−M−B bond angle and short B−B bond distance, in particular in cobalt bis(borylene) complexes, reveal the presence of a MB2 interaction consistent with some degree of weak B−B bonding. The π-bonding contribution is, in all complexes, smaller (28.4−32.6% of total orbital contributions) than the σ-bonding contribution. The BNX2 ligands are relatively poor π acceptors compared with the CO ligand, but better π acceptors than the PMe3 ligand. The contribution of M ← BNX2 ΔEσ is clearly the dominant term of the orbital interaction. The σ-donor ability of borylene ligands BNX2 is greater in bis(borylene) complexes [(η 5-C5H5)M(BNX2)2] than in carbonyl borylene complexes [(η 5-C5H5)(CO)M(BNX2)] and phosphine borylene complexes [(η 5-C5H5)(PM3)M(BNX2)2]. The absolute value of various energy terms for the MB bond decreases upon going from X = Me to SiH 3 and SiMe3. Table 1. Selected Structurally Characterized TwoCoordinated Terminal Metal−Borylene Complexes

1. INTRODUCTION Synthesis, structure, bonding, and reactivity of transition metal borylene complexes have been a provocative subject since the first report of structurally characterized terminal transition metal borylene complexes in 1998.1,2 So far a number of structurally characterized terminal transition metal borylene complexes have been reported (Table 1) and a number of interesting review articles have been written during these periods by Braunschweig et al.3−14 and Aldridge et al.15−18 Additionally, a number of basestabilized adducts formed by the coordination of a Lewis base (main group or transition metal) to the two- or three-coordinate boron center have also been reported.19,20 Theoretical approaches have been proven to be an indispensable part of the studies of terminal metal borylene complexes.21−32 Previously, the BLYP/LANL2DZ and B3LYP/LANL2DZ approaches have been applied to study the geometry and electronic structure of terminal cationic borylene complexes [(η 5-C5H5)(CO)2Fe{B(η 5-C5Me5)}]+, [(η 5-C5H5)(CO)2Fe(BMes)]+, [(η 5C 5 H 5 )(CO) 2 Fe(BNMe 2 )] + , and [(η 5 -C 5 H 5 )(CO) 2 Ru(BNMe2)]+.15,52 Structure and bonding energy analysis of transition metal borylene complexes have been studied in detail.28−31,33−35 To the best of our knowledge, the structure and MBR bonding analysis of the terminal neutral metal bis(borylene) complexes of cobalt, rhodium, and iridium have never been studied before. One should note that bis(borylene) complexes of cobalt and rhodium studied in this paper are not known so far. Only a single representative example of an iridium bis(borylene) complex, [(η 5-C5Me5)M{BN(SiMe3)2)}2], was reported by © 2011 American Chemical Society

complex [(η 5-C5H5)(CO)3V{BN(SiMe3)2}] [(CO)5Cr{BN(SiMe3)2}] [(CO)5Mo{BN(SiMe3)2}] [(CO)5W{BN(SiMe3)2}] [(Cy3P)(CO)4Cr{BN(SiMe3)2}] [(Cy3P)(CO)4Mo{BN(SiMe3)2}] [(Cy3P)(CO)4W{BN(SiMe3)2}] [(CO)5Cr{BSi(SiMe3)2}] [(η 5-C5H5)(CO)2Mn(BtBu)] [(Cy3P)2HClRu(BMes)] [(η 5-C5Me5)(CO)Ir{BN(SiMe3)2}] [(η 5-C5Me5)Ir{BN(SiMe3)2}2]

M−B (Å)

B−N/B− ∠M−B−N/ C (Å) M−B−C (deg)

1.959(6) 1.378(7) 1.996(6) 1.353(6) 2.125(2) 1.355(2) 2.151(7) 1.338(8) 1.915(2) 1.364(3) 2.059(3) 1.365(3) 2.058(6) 1.378(7) 1.878(10) 1.809(9) 1.531(11) 1.780(4) 1.558(5) 1.892(3) 1.365(4) 1.864(3) 1.398(3) 1.863(3) 1.393(3) [(η 5-C5Me5)(CO)2 Fe(BMes)]+ 1.792(8) 1.491(10) [(η 5-C5Me5)(CO)2 Fe(BNiPr2]+ 1.835(3) 1.334(3) [(η 5-C5Me5)(CO)2 Ru(BNiPr2]+ 1.950(8) 1.333(9) [(η 5-C5Me5)(CO)2 Fe(BNCy3]+ 1.859(6) 1.324(7) [(η 5-C5Me5)(CO)2 Fe(BNMe3]+ 1.811(3) 1.357(3) [(η 5-C5Me5)(CO)2 Ru(BNCy3]+ 1.860(6) 1.320(7) [(η 5-C5Me5)(PMe3((CO)Fe(BNCy3]+ 1.821(4) 1.347(5) [(η 5-C5Me5)(PMe3((CO)Ru(BNCy3]+ 1.928(4) 1.345(5) trans-[Br(PCy3)2Pt(BMes)] + 1.859(3) 1.495(4)

177.9(4) 177.4(4) 177.81(11) 177.9(5) 175.91(16) 175.3(2) 175.8(5) 174.3(7) 178.1(3) 175.9(3) 176.5(2) 178.6(2) 178.3(6)

175.4(2) 177.7(3) 170.8(3) 178.15(9)

Received: July 27, 2011 Published: October 21, 2011 5851

dx.doi.org/10.1021/om200695j | Organometallics 2011, 30, 5851−5858

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Braunschweig and co-workers.36 Structures of the other studied bis(borylene) complexes of iridium are hitherto unknown. We decided to investigate the nature of the MB bond in the terminal bis(borylene) complexes of cobalt, rhodium, and iridium with an energy decomposition analysis (EDA). Here, we report the geometry and electronic structure, as well as nature, of MBR bonds in the terminal neutral metal borylene complexes of cobalt, rhodium, and iridium complexes [(η 5-C5H5)M(BNMe2)2] (I, M = Co, II, M = Rh, III, M = Ir), [(η 5-C5H5)M{BN(SiH3)2)}2] (IV, M = Co, V, M = Rh, VI, M = Ir), and [(η 5-C5H5)M{BN(SiMe3)2)}2] (VII, M = Co, VIII, M = Rh, IX, M = Ir) at the DFT level using BP86/TZ2P/ZORA. The main goals of the present study are (i) to investigate the geometries and to analyze the nature of MB of the metal bis(borylene) complexes, (ii) to provide a quantitative differentiation of the M−BNX2 bonds, and (iii) to elucidate the role of the transition metal atoms and substituent X of BNX2 ligands in the stability of the M−BNX2 bond. The bonding situation in the molecules was investigated with the energy decomposition analysis, which has previously been used in systematic studies of transition metal complexes. 28−35 The EDA makes it possible to quantitatively estimate contributions of orbital interactions. We will discuss the degree of ionic and covalent character of the MB bonds as well as the extent of the M ← BNX2 σ-bonding and M → BNX2 π-back-bonding contribution to the M−BNX2 orbital interactions

The binding between the metal {[(η 5-C5H5)M(BNX2)] and borylene BNX2 fragments in complexes I−IX has been analyzed at the Cs symmetry using the energy decomposition scheme of the ADF package, which is based on the Morokuma44 and Ziegler and Rauk45 methods. On the basis of these studies, the bond energy ΔE between the fragments can be decomposed as (1) where ΔEprep is the energy required to promote of the free fragments from their equilibrium structure in the electronic ground state to that which they take up in the molecule:

(2) In eq 1, ΔEint is the instantaneous interaction energy between the two fragments of the molecule. It can be decomposed into three main components: (3) where ΔEelstat describes the classical Coulomb interaction between the fragments; ΔEPauli, which is called exchange repulsion or Pauli repulsion, takes into account the destabilizing two-orbital, three- or four-electron interactions between the occupied orbitals of both fragments; and ΔEorb is orbital interactions between the occupied and virtual orbitals of the two fragments. The electronic structures of the studied complexes were examined by Mayer46 and Nalewajski-Mrozek bond order47 and Hirshfeld atomic charges.48 All MO pictures were made by using the MOLDEN program.49

2. COMPUTATIONAL PROCEDURE Calculations of the neutral terminal bis(borylene) complexes [(η 5-C5H5)M(BNMe2)2] (I, M = Co, II, M = Rh, III, M = Ir), [(η 5-C5H5)M{BN(SiH3)2)}2] (IV, M = Co, V, M = Rh, VI, M = Ir), and [(η 5-C5H5)M{BN(SiMe3)2)}2] (VII, M = Co, VIII, M = Rh, IX, M = Ir) have been performed at the nonlocal DFT level of theory using the exchange functional of Becke37 and the correlation functional of Perdew38 (BP86). Scalar relativistic effects have been considered using the ZORA formalism.39 Uncontracted Slater-type orbitals (STOs) using triple-ζ basis sets augmented by two sets of polarization functions were employed for the SCF calculations.40 The (1s)2 core electrons of the boron, carbon, and nitrogen, (1s2s2p)10 core electrons of cobalt, (1s2s2p3s3p3d)28 core electrons of rhodium, and (1s2s2p3s3p3d4s4p4d)46 core electrons of iridium were treated by the frozen-core approximation.41 An auxiliary set of s, p, d, f, and g STOs was used to fit the molecular densities and to present the coulomb and exchange potentials accurately in each SCF cycle.42 The calculations were performed utilizing the program package ADF2010.02.43

3. RESULTS AND DISCUSSION 3.1. Geometries. The important bond distances and angles of the bis(borylene) complexes [(η 5-C5H5)M(BNX2)] I−IX along with those of the previously reported borylene complexes [(η 5-C5H5)(CO)Rh(BNX2)] (M = Co, Rh, Ir; X = Me, SiH3, SiMe3) and [(η 5-C5H5)(PMe3)Ir{BN(SiH3)2)}] calculated at the BP86/TZ2P level of theory are presented in Table 2. Geometric structures of [(η 5-C5H5)(PMe3)M{BN(SiMe3)2)}2] and [(η 5-C5H5)(L)Ir{BN(SiH3)2)}] (L = CO, PMe3, BN(SiH3)2) are shown in Figures 1 and 2, respectively. The optimized Cartesian coordinates of all studied complexes are given in the Supporting Information). Since bis(borylene) complexes of cobalt and rhodium are not known so far, here we report, for the first time in the literature, the structures of these cobalt and rhodium

Table 2. Selected Optimized Geometrical Parameters for Neutral Terminal Bis(borylene) Complexes [(η 5-C5H5)M(BNX2)2] (M = Co, Rh, Ir; X = Me, SiH3, SiMe3)a, I−IX [(η 5-C5H5)M(BNMe2)2] M = Co, I Bond Distances M−B 1.757 (1.767)b 1.758 B−N 1.396 (1.378) 1.396 B−B 1.982 Bond Angles M−B−N 175.4 (178.5) 175.8 B−M−B 68.7

[(η 5-C5H5)M{BN(SiH3)2}2]

[(η 5-C5H5)M{BN(SiMe3)2}2]

M = Rh, II

M = Ir, III

M = Co, IV

M = Rh, V

M = Ir, VI

1.866 (1.873) 1.864 1.390 (1.377) 1.391 2.139

1.880 (1.884) 1.877 1.388 (1.377) 1.388 2.357

1.747 (1.761) 1.748 [1.747]c 1.406 (1.389) 1.406 [1.407] 2.025

1.858 (1.878) 1.858 [1.849] 1.401 (1.386) 1.400 [1.407] 2.206

1.875 (1.886) 1.872 [1.861]{1.908}d 1.399 (1.386) 1.401 [1.406] {1.394} 2.370

177.8 (179.9) 177.8 (175.6) 176.2 (178.5) 177.8 178.8 176.2 [179.5] 70.0 77.7 70.8

M = Co, VII M = Rh, VIII 1.757 (1.780) 1.756 1.403 (1.382) 1.402 2.179

1.867 (1.885) 1.867 1.396 (1.378) 1.396 2.373

M = Ir, IX 1.882 (1.897) 1.881 1.396 (1.379) 1.396 2.517

177.7 (179.4) 178.6 (179.2) 173.9 (178.0) 175.5 (179.7) 176.3 (179.5) 177.7 [179.5] 178.7 [179.7] {175.3} 172.3 173.6 174.1 72.8 78.5{65.7} 76.7 79.9 84.0

a

Distances are in Å and angles are in deg. bCalculated data in parentheses for [(η 5-C5H5)(CO)M{BNX2].54 cCalculated data in brackets for [(η 5C5H5)(PMe3)M{BN(SiH3)2].54 dCalculated data in braces for [(η 5-C5H5)(PMe3)M{BN(SiH3)2] at BP86/TZ2P (nonrelativistic). 5852


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Figure 2. Optimized geometries of iridium borylene complexes [(η 5C5H5)(L)Ir{BN(SiH3)2}] (L = CO, PMe3, BN(SiH3)2).

single bond estimated on the basis of covalent radii predictions (Co−B = 1.95 Å, Rh−B = 2.10 Å, Ir−B = 2.07 Å). 50 Using the relationship between the bond order and bond distance suggested by Pauling,51 the Pauling bond order of the optimized M−B bond distances in these complexes is 1.93 (I); 2.14 (II); 1.85 (III); 1.99 (IV); 2.19 (V); 1.88 (VI); 1.93 (VII); 2.13 (VIII), and 1.84 (IX). Thus, the M−B bonds in the complexes I−IX are nearly MB double bonds. Upon going from M = Co to M = Ir, the calculated MB bond distance increases in the order Co < Rh < Ir. On substitution of the BNX2 ligand by the more π-acidic CO ligand, the calculated M−B bond distances increase, while substitution of the BNX 2 ligand by the less π-acidic PMe3 ligand leads to a decrease of the calculated M−B bond distances (see Table 2). Moreover, the nature of the substituent X of the ligand BNX2 has an insignificant effect on the nature of the MBNX2 bonding:

Figure 1. Optimized geometries of bis(borylene) complexes [(η 5C5H5)M{BN(SiMe3)2}2] (III, M = Co; VI, M = Rh; IX, M = Ir) The important bond lengths and angles are given in Table 2.

bis(borylene) complexes. As summarized in Table 2, the calculated geometry parameters of iridium bis(borylene) complex [(η 5-C5H 5)Ir{BN(SiMe 3) 2)}2] are in excellent agreement with their available experimental values for [(η 5C5Me5)Ir{BN(SiMe3)2)}2] (Table 1).36 We expect the same accuracy for the other studied metal−bis(borylene) complexes of Co, Rh, and Ir. As seen in Table 2, the M−B bond distances in the studied complexes (I−IX) are shorter than those expected for a M−B 5853


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Figure 3. Spatial plot of the three-center, two-electron bonding molecular orbital showing the weak interaction between M and the two boron centers.

complemented with a numerical analysis of the energy decomposition analysis. An energy decomposition analysis of the MB bonds in the calculated metal−bis(borylene) complexes [(η 5-C5H5)M(BNX2}2] (M = Co, Rh, Ir; X = Me, SiH3, SiMe3) is summarized in Table 3 and Figure 5. The calculated results for bis(borylene) complexes I−IX are compared with the previously reported EDA results for borylene complexes [(η 5C5H5)(CO)M(BNX2] (X = Me, SiH3, SiMe3) and [(η 5C5H5)(PMe3)M{BN(SiH3)2}] (M = Co, Rh, Ir). The tabulated bond dissociation energies (BDEs) in Table 3 reveal the expected periodic trend in bond strengths due to dorbital extent: the IrB bonds are stronger than those for corresponding cobalt and rhodium complexes. The interaction energies, ΔEint, show the same trend as the calculated BDEs, with the discrepancies between the two values (i.e., ΔEprep) amounting to 5.6−13.6 kcal/mol. Figure 5 shows schematically the variation in BDE, Pauli repulsion, ΔEPauli, π-bonding, interaction energies, ΔEint, orbital interactions, ΔEorb, and electrostatic interactions, ΔEelstat, for bis(borylene) complexes I−IX. The breakdown of the interaction energy ΔEint into the repulsive term ΔEPauli and the attractive terms ΔEorb and ΔEelstat shows that ΔEPauli repulsive interactions have the larger absolute values for the studied complexes I−IX (Table 3, Figure 5). The contributions of the electrostatic interaction terms ΔEelstat are significantly larger in all bis(borylene) complexes (I−IX) than the covalent bonding ΔEorb term. Thus, the [M]BR bond in the studied bis(borylene) complexes of Co, Rh, and Ir has a greater degree of ionic character (59.3−67.6%), in agreement with previous studies for borylene complexes of cobalt, rhodium, and iridium. Table 3 also gives the breakdown of the orbital interactions ΔEorb into the M ← BNX2 σdonation and M → BNX2 π-back-donation components. It is significant to note that the π-bonding contribution is, in all complexes, smaller (28.4−32.6% of total orbital contributions) than the σ-bonding contribution. As compared to bis(borylene) complexes [(η 5-C5H5)M(BNX2)2], the relatively smaller π contributions are found for the complexes [(η 5-C5H5)(CO)M(BNX2)], with a larger π contribution being observed for the complexes [(η 5-C5H5)(PMe3)M{BN(SiH3)2}]. Thus, BNX2 ligands are relatively poor π acceptors compared with the CO ligand, but better π acceptors than the PMe3 ligand. The contribution of M ← BNX2 ΔEσ is clearly the dominant term of

The MB bond distance in bis(borylene) complexes I−IX only slightly increases upon going from Me to SiMe3 and SiH3. The B−N optimized bond distances, 1.388−1.406 Å, in bis(borylene) complexes I−IX are also shorter than that expected for the single B−N bond based on covalent radii predictions (B−N = 1.56 Å).50 The Pauling bond order of the optimized B−N bond distances in these complexes are in the range 1.65−1.75 for I−IX. The B−N bonding in bis(borylene) complexes [(η 5-C5H5)Ir(BNX2)2] is relatively weaker than the B−N bonding in the borylene complexes [(η 5-C5H5)(CO)Ir(BNX2)]. The bis(borylene) complexes I−IX have B−M−B angles in the range 68.7−84.0° and B−B distances in the range 1.982− 2.517 Ǻ (Table 2), indicating that the acute B−M−B bond angle and short B−B bond distance, in particular in cobalt bis(borylene) complexes, reveal the presence of a MB2 interaction consistent with some degree of weak B−B bonding. An envelope plot of an orbital showing the interaction between a metal and the two boron centers is presented in Figure 3. Marder and co-workers reported similar CoB2 interactions in cobalt bis(boryl) complexes.52−54 Upon going from M = Co to M = Ir, the B−B bond distance and, thus, the B−B interaction decreases (Table 2, Figure 3). The B−B distance also decreases upon changing the X group of BNX2 from Me to SiH3 and SiMe3. The Pauling bond orders corresponding to B−B distances (sum of single B−B bond distance, 1.70 Ǻ ) are presented in Figure 1S (see Supporting Information). The results reveal that complex [(η 5-C5H5)Ir{BN(SiMe3)2}2] has the weakest B−B interaction. 3.2. Energy Decomposition Analysis of the MB Bonding of the Bis(borylene) Complexes I−IX. It is instructive for the numerical analysis of the metal−borylene bonding analysis to present and to briefly discuss the molecular orbitals of the model bis(borylene) complex [(η 5-C5H5)Ir{BN(SiH3)2}2] (VIII). Figure 4 displays the six occupied MOs of VIII relevant for the Ir−B and B−N bonding. The shape of the valence orbitals indicates that there are five occupied MOs that contribute to the [Ir]−BNX2 bond. The HOMO is a three-center, B−Ir−B π-bonding orbital. The HOMO−4 and HOMO−9 orbitals have small contributions to Ir−B π bonding. HOMO−11 and HOMO−14 depict Ir−B σ bonding. The HOMO−5 orbital is a B−N π-bonding orbital. The pictorial representation of the molecular orbital will now be 5854


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Figure 4. Plot of some relevant molecular orbitals of the iridium bis(borylene) complex [(η 5-C5H5)Ir{BN(SiH3)2)}2].

the orbital interaction, and the EDA data thus suggest that the BNX2 ligands behave predominantly as σ donors. Moreover, the σ-donor ability of borylene ligands BNX2 is greater in bis(borylene) complexes [(η 5-C5H5)M(BNX2)2] than in carbonyl borylene complexes [(η 5-C5H5)(CO)M(BNX2)] and phosphine borylene complexes [(η 5-C5H5)(PM3)M(BNX2)2]. Furthermore, the nature of the substituent X of ligand BNX2 has little effect on the MBNX2 bonding: the absolute value of various energy terms for the MB bond decreases upon going from X = Me to SiH3 and SiMe3. For comparing the results of relativistic and nonrelativistic calculations, we also carried out purely nonrelativistic calculations for [(η 5-C5H5)Ir{BN(SiH3)2}2] at the BP86/ TZ2P level. The optimized structure is presented in the Supporting Information (Figure 2S). Optimized geometrical parameters as well as energy values are given in Tables 2 and 3,

respectively. The relativistic effects are less important for cobalt and rhodium. Relativistic effects destabilized the 5d orbitals of iridium due to relativistic contraction of the s and p core orbitals, which in turn reduces the effective nuclear charge experienced by electrons in the 5d shell on iridium. Hence, the relativistic effects increase the metal−ligand interactions as well as the bond strengths. It follows from Tables 2 and 3 that relativity shortens Ir−B bond distances (0.033 Å) and increases bond dissociation energy (5.4 kcal/mol) compared to the calculated data for nonrelativistic calculations for complex [(η 5C5H5)M{BN(SiH3)2}2]. 3.3. Bonding Analysis of the MBNX2 Bonds of the Complexes I−IX. In addition to an energy decomposition analysis of the MB bonds, we begin the analysis of the MB bonding in the complexes I−IX with a discussion of bond orders: Mayer bond order,46 Nalewajski-Mrozek bond order,47 5855


Energy contributions in kcal/mol. bCalculated data for [(η 5-C5H5)(CO)M{BNX2].54 cCalculated data for [(η 5-C5H5)(PMe3)M{BN(SiH3)2].54 dThe values of percentage contribution to the total electrostatic interactions reflecting the ionic character of the bond. eThe values of π bonding to the total orbital interactions ΔEorb. fBond dissociation energy with negative sign. gCalculated values at BP86/ TZ2P (non-relativistic).

ΔEprep ΔE(−BDE)f

ΔEorb ΔEσ(a′) ΔEπ(a′′)e

Figure 5. Values of the energy contributions of bond dissociation energy, π-bonding energy, interaction energy, orbital interactions (covalent interaction), and electrostatic interactions (ionic contribution) to the M−B bonding in terminal bis(borylene) complexes [(η 5C5H5)M(BNX2)2] (M = Co, Rh, Ir; X = Me, SiH3, SiMe3), I−IX.

and Hirshfeld atomic charges.48 The results of Pauling, Mayer, and Nalewajski-Mrozek bond orders for bis(borylene) complexes I−IX are presented in Table 4. The basis set independent Pauling bond orders are better, in particular for weak B−B bond order, than the basis set dependent Mayer and Nalewajski-Mrozek bond orders. A seen from Table 4, the values of the different bond orders of the MB bonds in bis(borylene) complexes I−IX suggest significant covalent contributions to the M−B bonds. Trends of Mayer and Nalewajski-Mrozek bond orders for the MB bond are the same. The values of the MB bond orders vary as Ir > Rh < Co. Thus, the RhB bond orders are smaller than the CoB and IrB bond orders in all three sets of complexes I− III, IV−VI, and VII−IX. The values for the B−N bond order suggest some multiple bonding in the B−N bonds of the complexes I−IX. The calculated Hirshfeld charge distributions are presented in Figure 6. The results indicate that the C5H5 ligands carry negative charge (−0.02 to −0.22) in these bis(borylene) complexes with greater negative charge in rhodium complexes [(η 5-C5H5)Rh{BNX2] than in cobalt and iridium complexes [(η 5-C5H5)M{BNX2] (M = Co, Ir). The BNX2 groups are positively charged. The cobalt and iridium atoms carry negative charge, while rhodium atoms are positively charged. The abnormal trend for rhodium bis(borylene) complexes may be due to the different electronic configuration of Rh than Co and Ir.

4. CONCLUSIONS A theoretical study has been presented where the structure and bonding situation in neutral terminal bis(borylene) complexes of cobalt, rhodium, and iridium [(η 5-C5H5)M(BNX2)] (X = Me, SiH3, SiMe3) are investigated for the first time (except Ir complexes IX). The calculated geometry of iridium bis(borylene) complex [(η 5-C5H5)Ir{BN(SiMe3)2}2] is in excellent agreement with structurally characterized iridium complex [(η 5-C5Me5)Ir{BN(SiMe3)2}2]. The M−B bonds in the complexes (I−IX) are nearly MB double bonds. On substitution of the BNX2 ligand by the more π-acidic CO ligand, the calculated M−B bond distances increase, while substitution of the BNX2 ligand by the less π-acidic PMe3 ligand results in the calculated M−B bond distances decreasing. The acute B−M−B bond angle and short B−B bond distance, in particular in cobalt bis(borylene) complexes, reveal the presence of a MB2 interaction, consistent with some degree of weak B−B bonding.

a

M = Ir, IX

−124.2 (−122.0) 341.6 (328.8) −314.9 (−309.3) 67.6% (68.5%) −150.9 (−142.5) −108.1 (−105.1) −42.8 (−37.4) 28.4% (26.2%) 13.6 (10.7) −110.6 (−111.3)

M = Rh, VIII M = Co, VII M = Ir, VI M = Rh, V

−106.6 (−99.0) [−102.5] 306.5 (264.3) [281.2] −270.0 (−241.2) [−254.0] 65.4% (66.4%) [66.2%] −143.1 (−122.2) [−129.7] −101.2 (−86.2) [−86.0] −41.9 (−36.0) [−43.7] 29.3% (29.5%) [33.7%] 8.5 (5.5) [8.7] −98.1 (−93.5) [−93.8]

M = Co, IV

−110.9 (−98.2) [−99.4]c 283.9 (223.6) [223.0] −234.1 (−197.5) [−197.2] 59.3% (61.4%) [61.2%] −160.6 (−124.2) [−125.2] −113.6 (−86.3) [−78.9] −47.1 (−37.9) [−46.3] 29.3% (30.5%) [37.0%] 7.5 (1.4) [0.8] −103.4 (−96.8) [−98.6]

M = Ir, III M = Rh, II

−108.8 (−102.3) 324.5 (283.3) −286.1 (−259.9) 66.0% (67.4%) --147.2 (−125.7) −103.7 (−88.1) −43.5 (−37.6) 29.6% (29.9%) 8.5 (6.0) −100.3 (−96.3)

M = Co, I

−127.3 (−125.0) 355.5 (334.7) −323.5 (−312.3) 67.0% (67.9%) −159.3 (−147.4) −113.3 (−104.2) −46.0 (−43.2) 28.9% (29.3%) 11.5 (10.4) −115.8 (−114.6)

−125.6 (−122.1) [−126.4] {−116.8}g 339.3 (319.2) [336.7] {372.4} −306.3 (−294.7) [−307.6] {318.0} 65.9% (66.8%) [66.4%] −158.6 (−146.6) [−155.4]{−171.3} −113.3 (−104.5) [−105.5] {−127.5} −45.3 (−42.1) [−49.9] {−43.8} 28.6% (28.7%) [32.1%] 11.0 (10.2) [13.8] {7.6} −114.6 (−111.9) [−112.6] {−109.2}

−105.9 (−97.9) 265.2 (229.6) −225.6 (−206.5) 60.8% (63.1%) −145.5 (−121.0) −97.8 (−87.7) −47.7 (−33.3) 32.8% (27.5%) 8.5 (1.8) −97.4 (−96.1)

−103.9 (−99.0) 300.4 (279.9) −272.2 (−258.0) 67.3% (68.1%) −132.1 (−120.9) −93.5 (−88.9) −8.6 (−32.0) 29.2% (26.5%) 9.9 (5.7) −94.0 (−93.3)

Article

−110.7 (−101.3)b 304.5 (231.4) −249.8 (−207.5) 60.2% (62.4%) −165.4 (−125.2) −116.4 (−86.0) −49.0 (−39.2) 29.6% (31.3%) 5.6 (2.5) −105.1 (−98.8) ΔEint ΔEPauli ΔEelstatd

[(η 5-C5H5)M{BN(SiMe3)2}2] [(η 5-C5H5)M{BN(SiH3)2}2] [(η 5-C5H5)M(BNMe2)2]

Table 3. Energy Decomposition Analysisa of Neutral Terminal Bis(borylene) Complexes [(η 5-C5H5)M(BNX2)2] (M = Co, Rh, Ir; X = Me, SiH3, SiMe3) at BP86/TZ2P, I−IX

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Table 4. M−B, B−N, and B−B Bond Orders in Neutral Terminal Bis(borylene) Complexes [(η 5-C5H5)M(BNX2)2] (M = Co, Rh, Ir; X = Me, SiH3, SiMe3) bond orders Pauling 5

[(η -C5H5)Co(BNMe2)2] [(η 5-C5H5)Rh(BNMe2)2] [(η 5-C5H5)Ir(BNMe2)2] [(η 5-C5H5)Co{BN(SiH3)2}2] [(η 5-C5H5)Rh{BN(SiH3)2}2] [(η 5-C5H5)Ir{BN(SiH3)2}2] [(η 5-C5H5)Co{BN(SiMe3)2}2] [(η 5-C5H5)Rh{BN(SiMe3)2}2] [(η 5-C5H5)Ir{BN(SiMe3)2}2]

Mayer

Nalewajski-Mrozek

M−B

B−N

B−B

M−B

B−N

B−B

M−B

B−N

B−B

1.93 1.93 2.14 2.15 1.85 1.87 1.99 2.00 2.19 2.19 1.88 1.90 1.93 1.94 2.13 2.13 1.84 1.85

1.70 1.70 1.74 1.73 1.75 1.75 1.65 1.65 1.67 1.68 1.67 1.69 1.66 1.67 1.70 1.70 1.70 1.70

0.40

1.45 1.44 1.34 1.33 1.68 1.67 1.47 1.47 1.31 1.32 1.67 1.68 1.55 1.54 1.36 1.34 1.69 1.68

1.19 1.20 1.20 1.20 1.22 1.23 1.02 1.02 1.02 1.02 1.05 1.04 1.07 1.09 1.07 1.10 1.08 1.09

0.56

1.46 1.45 1.34 1.34 1.39 1.39 1.34 1.34 1.39 1.39 1.60 1.62 1.47 1.47 1.35 1.35 1.62 1.61

1.28 1.28 1.35 1.34 1.58 1.58 1.26 1.26 1.58 1.58 1.21 1.20 1.23 1.22 1.33 1.32 1.27 1.26

0.64

0.24 0.12 0.35 0.19 0.11 0.21 0.11 0.07

0.59 0.27 0.54 0.56 0.27 0.43 0.49 0.21

0.49 0.51 0.44 0.51 0.33 0.48 0.49 0.27

Figure 6. Hirshfeld charge distributions in bis(borylene) complexes [(η 5-C5H5)M{BNX2] (M = Co, Rh, Ir; X = Me, SiH3, SiMe3), I−IX.

and phosphine borylene complexes [(η 5-C5H5)(PM3)M(BNX2)2]. The nature of the substituent X of ligand BNX2 has little effect on the MBNX2 bonding: the absolute value of various energy terms for the MB bond decreases upon going from X = Me to SiH3 and SiMe3. We believe that a more detailed understanding of the bonding in metal−borylene complexes is a requisite, particularly for the synthesis of terminal transition metal borylene complexes. In this aspect, the above presented findings are important contributions to the fast developing metal−borylene chemistry.

The π-bonding contribution is, in all bis(borylene) complexes, smaller (28.4−32.6% of total orbital contributions) than the σ-bonding contribution. The BNX2 ligands are relatively poor π acceptors compared with the CO ligand, but better π acceptors than the PMe3 ligand. The contribution of M ← BNX2 ΔEσ is clearly the dominant term of the orbital interaction, and the EDA data thus suggest that the BNX2 ligands behave predominantly as σ-donors. Moreover, the σdonor ability of borylene ligands BNX 2 is greater in bis(borylene) complexes [(η 5-C5H5)M(BNX2)2] than in carbonyl borylene complexes [(η 5-C5H5)(CO)M(BNX2)] 5857


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ASSOCIATED CONTENT S Supporting Information * Cartesian coordinates of the optimized geometries of metal bis(borylene) complexes (I−IX), Figure 1S and Figure 2S. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

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REFERENCES

Corresponding Author *E-mail: [email protected].

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Bis(borylene) Complexes of Cobalt, Rhodium, and Iridium - American

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