Binuclear Alkaline Earth Metal Compounds (Be, Mg, Ca, Sr, Ba) with α

ARTICLE pubs.acs.org/Organometallics

Binuclear Alkaline Earth Metal Compounds (Be, Mg, Ca, Sr, Ba) with r-Diimine Ligands: A Computational Study Shaoguang Li,†,§ Xiao-Juan Yang,*,† Yanyan Liu,† Yanxia Zhao,†,§ Qian-Shu Li,# Yaoming Xie,^ Henry F. Schaefer,^ and Biao Wu† †

State Key Laboratory for Oxo Synthesis & Selective Oxidation, Lanzhou Institute of Chemical Physics, CAS, Lanzhou 730000, China Center for Computational Quantum Chemistry, School of Chemistry and Environment, South China Normal University, Guangzhou 510631, China ^ Center for Computational Chemistry, The University of Georgia, Athens, Georgia 30602, United States § Graduate University of Chinese Academy of Sciences, Beijing 100049, China #

bS Supporting Information ABSTRACT: A series of dinuclear alkaline earth metal (Ae) structures Na2[(CHNH)2Ae
Ae(CHNH)2] (1
5) and (CHNH)2Ae
Ae(CHNH)2 (6
10) containing Ae
Ae bonds were optimized using density functional theory (DFT) and hybrid Hatree-Fock/DFT methods. The geometric and NBO analyses indicate that there are covalent Ae
Ae bonds in these R-diimine-stabilized structures, which have been further investigated by NBO second-order interaction and bond dissociation energetic examinations.

’ INTRODUCTION The extraordinary synthesis of the first stable compound containing a Zn
Zn bond, Cp*Zn
ZnCp* (Cp* = C5Me5), in 2004 by Resa, Carmona, Gutierrez-Puebla, and Monge1 has motivated much interest in the study of analogous Cp2M2 compounds in both experimental and theoretical aspects.2
8 Given the similarities of their electronic configurations of group 2 and 12 elements, attention has also focused on group 2 metals. However, the alkaline earth metals (Ae) are regarded as less redox-active, and the oxidation state of þ2 dominates their chemistry, while low oxidation states are much less common among Ae compounds.9
13 Following the report in Science of the Cp*Zn
ZnCp* compound,1 theoretical studies have predicted that some Ae compounds in their low oxidation states are viable molecules, for example the analogues of binuclear metallocenes. Xie, Schaefer, and King14 first studied the electronic structure and energetics of the binuclear metallocenes CpMMCp (M = Be, Mg, and Ca). A series of dimeric compounds RMMR [R = (BCO)5, (BNN)5; M = Be, Mg, Ca, Zn, Cd] were investigated by Li and co-workers.15 Although experimental approaches to stable binuclear metallocenes have not so far been successful for Ae elements, in 2007 Green, Jones, and Stasch16 reported in Science two molecular Mg
Mg-bonded compounds with the magnesium metal in the þ1 oxidation state, RMgMgR (R1 = [(Ar)NC(NiPr2)N(Ar)]
; r 2011 American Chemical Society

R2 = {[(Ar)NC(Me)]2CH}
, where Ar is 2,6-diisopropylphenyl). This achievement confirmed the theoretical prediction of Mg
Mg bond formation. The experimental bond distances and the calculated bond dissociation energies for these compounds fall in the range of the theoretical studies for Mg
Mg bonds. An “inverse” sandwich complex, [(THF)3Ca{μ-C6H3-1,3,5-Ph3}Ca(THF)3], reported by Krieck and co-workers17 in 2009 provided another important case of stable alkaline earth metal compounds in the þ1 oxidation state. Recently, we have used bulky R-diimine ligands to stabilize a series of metal
metal-bonded compounds M0 2[LMI
MIL] (M = Zn, Mg; M0 = Na, K) and [LZnI
ZnIL].18
22 In the [M0 (THF)n]2[LM
ML] (L = [(2,6-iPr2C6H3)NC(Me)]22
) complexes,21 the Mg2þ or Zn2þ ions are reduced to the low oxidation state Mþ, while the R-diimine ligands are reduced to the dianionic form. Notably, the negative charges in the reduced R-diimine ligand are further compensated by a solvated Naþ or Kþ ion, which is π-bonded to the N
CdC
N moiety of the ligand. In the [LZnI
ZnIL] (L = [(2,6-iPr2C6H3)NC(Me)]2
) complex, the Znþ ion was stabilized by the monoanionic Rdiimine ligands.22 We have carried out theoretical studies on these Zn
Zn- or Mg
Mg-bonded compounds;19,21 however, Received: March 5, 2011 Published: May 03, 2011 3113

dx.doi.org/10.1021/om200203n | Organometallics 2011, 30, 3113–3118

Organometallics

ARTICLE

Figure 1. B3LYP and BP86 (italic font) optimized equilibrium geometries of the compounds Na2[(CHNH)2Ae
Ae(CHNH)2]. Bond distances are in Å.

Figure 2. B3LYP and BP86 (italic font) optimized equilibrium geometries of investigated compounds (CHNH)2Ae
Ae(CHNH)2. Bond distances are in Å.

no further theoretical insights into related aspects have been addressed so far, such as how the metal
metal bond is affected by the ligand status (dianionic or monoanionic) and whether it is possible to synthesize other Ae
Ae-bonded compounds with these R-diimine ligands. Herein we report a theoretical investigation on a series of neutral molecules [Na]2[(CHNH)2Ae
Ae(CHNH)2] (structures 1 to 5, Ae = Be, Mg, Ca, Sr, Ba; Figure 1) as well as (CHNH)2Ae
Ae(CHNH)2 without the alkali metal ions (6
10, Figure 2). In these model compounds, the oxidation state of the alkaline earth metals is þ1, while the valence status of the R-diimine ligands is
2 in structures 1 to 5 and
1 in structures 6 to 10. NBO and energetic analyses demonstrate that the Ae
Ae bonds are strongly stabilized by the [Na
R-diimine]
groups (structures 1
5) compared with the

[R-diimine]
ligands (6
10). These computations extend the new bis-metallic complexes of Ae elements beyond the metallocenes, which not only can contribute to a deeper understanding of the Ae
Ae bonds but also may provide guidelines for future synthetic work in this field.

’ THEORETICAL METHODS Our initial model molecule, [Na]2[(CHNH)2Ae
Ae(CHNH)2], was simplified from the crystal data of [K(THF)3]2[LMg
MgL] (L = [(2,6-iPr2C6H3)NC(Me)]22
)21 through substitution of the N-2, 6-diisopropylphenyl and methyl groups in the ligand L by H atoms, as well as replacing the solvated K by Na atoms. Such alternations should not qualitatively affect the validity of the computational results with respect to the properties of the metal
metal bonds. 3114

dx.doi.org/10.1021/om200203n |Organometallics 2011, 30, 3113–3118

Organometallics The equilibrium geometrical structures were optimized by the density functional theory (DFT) or hybrid Hatree-Fock/DFT methods, which have been widely recommended as practical and effective computational tools, especially for organometallic compounds.18
24 The B3LYP method incorporates Becke’s three-parameter functional (B3) with the Lee, Yang, and Parr (LYP) correlation functional.25,26 Our second approach is the BP86 method, which combines Becke’s 1988 exchange functional with Perdew’s 1986 correlation functional.27,28 The def2-TZVPP basis sets were used for all alkaline earth metals.29,30 For sodium the McLean and Chandler (12s9p/6s4p) basis set was appended with a set of d-type polarization functions Rd(Na) = 0.175.31 The DZP basis sets for carbon, nitrogen, and hydrogen in this paper were constructed by augmenting the Huzinaga
Dunning contracted doubleζ basis with one set of five d-type polarization functions for each C and N atom and a set of p functions on each H atom. The final set for C and N contains 15 contracted functions and are denoted (9s5p1d/4s2p1d). Finally, there are five functions per H atom (4s1p/2s1p).32
34 Natural bond orbital (NBO) analyses are performed with the B3LYP method, to gain insight into the bonding patterns of these compounds.35 All computations were carried out using the GAUSSIAN 03 program suite.36

’ RESULTS AND DISCUSSION Geometric Structures. The computed vibrational frequencies revealed that all structures are minima on their respective potential energy surfaces. Figures 1 and 2 show the optimized equilibrium structures of the [Na]2[(CHNH)2Ae
Ae(CHNH)2] (1
5, Ae = Be, Mg, Ca, Sr, Ba) and (CHNH)2Ae
Ae(CHNH)2 (6
10, Ae = Be, Mg, Ca, Sr, Ba) complexes at the B3LYP and BP86 levels, respectively. In the structure of Na2[(CHNH)2Be
Be(CHNH)2] (1) with the C2h symmetry, the Be
Be distance is predicted to be 2.211 Å (B3LYP) or 2.214 Å (BP86). For comparison, in the structure (CHNH)2Be
Be(CHNH)2 (6), which is also C2hsymmetric, the computed Be
Be distance is 2.158 Å (B3LYP) or 2.148 Å (BP86). These distances are slightly longer than those predicted for HBe
BeH (2.095 or 2.113 Å, at the B3LYP/DZP or BP86/DZP level, respectively), [C6H5Be]2 (2.095 Å with BP86/TZVPP), and CpBe
BeCp (2.057 or 2.066 Å, at the B3LYP/DZP or BP86/DZP level; 2.081 Å, at the BP86/TZ2P level).13,14,37 However, all our reported Be
Be distances are distinctly less than that (2.45 Å) recently reported for the isolated Be2 molecule,37 for which the interaction falls short of a single bond. The structure of the analogous magnesium compound Na2[(CHNH)2Mg
Mg(CHNH)2] (2), with C2 (B3LYP) or Ci (BP86) symmetry, shows the Mg
Mg distance to be 2.887 Å (B3LYP) or 2.962 Å (BP86), while the C2v (B3LYP) or D2h (BP86) structures of 7, (CHNH)2Mg
Mg(CHNH)2, give Mg
Mg distances of 2.820 Å (B3LYP) or 2.843 Å (BP86). These are comparable to results for CpMg
MgCp (2.766 or 2.786 Å, at the B3LYP/DZP or BP86/DZP level; 2.790 Å, at the BP86/TZ2P level).13,14 The earlier theoretical study of HMg
MgH yielded an Mg
Mg distance of 2.862 Å (B3LYP/ DZP) or 2.844 Å (BP86/DZP).14 The calculated Mg
Mg distance for [C6H5Mg]2 is 2.870 Å (BP86/TZVPP).37 In previous work based on the model structure K2[(CHNH)2Mg
Mg(CHNH)2] performed at the B3LYP/DZP level, the Mg
Mg distance is 2.906 Å, which agreed well with the experimental distance 2.937 Å.21 The geometry parameters in the present study at the same level of theory should also be reliable.

ARTICLE

The optimized dicalcium(I) compounds 3 (C2 symmetry) and 8 (C2h symmetry) have Ca
Ca distances of 3.864 (B3LYP) or 3.870 Å (BP86) and 3.905 (B3LYP) or 3.906 Å (BP86), respectively. These distances are roughly 0.1 Å longer than those for HCa
CaH (3.794 or 3.797 Å, B3LYP/DZP or BP86/DZP level), CpCa
CaCp (3.740 or 3.734 Å, B3LYP/DZP or BP86/ DZP level; 3.823 Å, BP86/TZ2P level), and [C6H5
Ca]2 (3.806 Å, BP86/TZVPP level), indicating slightly weaker metal
metal bonds.13,14,37 In the heavier dimetallocene compounds, the Sr
Sr and Ba
Ba distances were predicted to be 4.171 and 4.662 Å (at BP86/TZ2P level), respectively.13 In the related heavy diphenyl dialkaline earth metal(I) compounds H5C6Ae
AeC6H5, the Sr
Sr and Ba
Ba distances were reported to be 4.121 and 4.537 Å (BP86/TZVPP level).37 Our research for the analogous Sr
Sr distances in structures 4 and 9 gives the values 4.238/ 4.296 and 4.307/4.279 Å, while the Ba
Ba distances in 5 and 10 are 4.889/4.867 and 4.962/4.859 Å. The metal
metal and the metal
N bond distances increase monotonically from Be to Ba. In the Na2[(CHNH)2Ae
Ae(CHNH)2] models, only the lightest Be atom is coplanar with the NCCN plane of the R-diimine ligand, while the other Ae atoms deviate from the R-diimine plane gradually (Figure 1). All the other Na2[(CHNH)2Ae
Ae(CHNH)2] complexes show trans bent arrangements, especially for the calcium, strontium, and barium derivatives (Figure 1). In contrast, for the (CHNH)2Ae
Ae(CHNH)2 models 6
10 (Figure 2) with the monoanionic ligands and without Na atoms, all the Ae metals are coplanar with the R-diimine ligand, and the metal
N bond distances are shorter than for the alkali-metal-stabilized complexes 1
5. It is interesting that in the lighter Ae
Ae complexes (Be and Mg) the metal
metal bonds in the Na2[(CHNH)2Ae
Ae(CHNH)2] species are slightly longer than those in (CHNH)2Ae
Ae(CHNH)2 models, but the homologous heavier element Ca, Sr, and Ba derivatives show opposite features. It is known that the neutral R-diimine ligands can be reduced to the monoanionic or dianionic form,38 and the C
C and C
N bond lengths change accordingly (elongation of C
N bonds and shortening of C
C bond). These structural changes can be used to distinguish the status of the ligand. In the Na2[(CHNH)2Ae
Ae(CHNH)2] structures 1
5, the C
C bonds of the R-diimine ligand are in the range 1.375
1.384 Å (B3LYP) or 1.388
1.398 Å (BP86), and the corresponding C
N bonds are 1.405
1.416 Å (B3LYP) or 1.408
1.422 Å (BP86). For the (CHNH)2Ae
Ae(CHNH)2 models 6
10, the C
C bonds are optimized to be 1.391
1.426 Å (B3LYP) or 1.405
1.432 Å (BP86), while the C
N bonds are 1.334
1.378 Å (B3LYP) or 1.349
1.380 Å (BP86). According to previous experimental18
22 and theoretical21,22 studies, the R-diimine ligands within models 1
5 are dianions, and those in 6
10 are monoanions. The NBO analysis also supports these assignments (see below). NBO Analysis. The Wiberg bond indices (WBIs) of the Ae
Ae bonds and the natural charges on selected atoms for Na2[(CHNH)2Ae
Ae(CHNH)2] (1
5) and (CHNH)2Ae
Ae(CHNH)2 (6
10) are reported in Table 1. The WBI values for the Ae
Ae bond in the Na2[(CHNH)2Ae
Ae(CHNH)2] (1
4; Ae = Be, Mg, Ca, Sr) species range from 0.81 to 0.94, indicating the existence of a covalent metal
metal single bond in each compound. The heaviest element Ba compound displays a smaller bond order (0.55). The natural charges on the sodium atom in these species are larger than 0.80, and the WBI values between the Na atom and the atoms in the R-diimine ligand are 3115

dx.doi.org/10.1021/om200203n |Organometallics 2011, 30, 3113–3118

Organometallics

ARTICLE

predicated to be less than 0.1, which implies that the bonding between Na and the ligand is predominantly ionic in nature and the electrostatic interaction dominates the Na
ligand interaction, with one electron of the Na atom transferred to other parts of the compound. For the complex Na2[(CHNH)2Ba
Ba(CHNH)2] (5), the sodium atom in the [LNa]
fragment adopts an η2-coordination mode with the ligand, and the natural charges located on sodium decrease to 0.62. This weaker ionic bonding may be related to the less covalent bonding between the two Ba atoms. In the series (CHNH)2Ae
Ae(CHNH)2 (6
10) without the sodium ions, the Mg
Mg Wiberg bond index approaches 1.0 (0.96), but the WBI values for other metal
metal bonds are quite small, especially for the first and last elements, Be (0.36) and Ba (0.33). Furthermore, the Ae
Ae interactions in species 6
10 are much weaker than those in their alkali-metal-stabilized counterparts (1
5), indicating that the covalent characters of these metal
metal bonds are smaller. Thus it appears that the existence of the Na atoms strengthens the Ae
Ae covalent bonding. The analysis of the NBO second-order interaction energies gives a further interpretation to the stabilizing effect of the Na atoms, which are calculated by the following equation: ΔEij ¼ qi

Fði, jÞ2 εi
εj

where qi is the donor orbital occupancy, εi and εj are diagonal elements (orbital energies) of the Fock matrix, and F(i,j) is the off-diagonal NBO Fock matrix element. The stabilization energy Table 1. B3LYP-Computed Wiberg Bond Indices (WBIs) of the Ae
Ae Bond and Natural Charges on Ae and Na Atoms species

WBIAe
Ae

qAe

qNa

Na2[(CHNH)2Be
Be(CHNH)2] (1)

0.806

0.338

0.836

Na2[(CHNH)2Mg
Mg(CHNH)2] (2) Na2[(CHNH)2Ca
Ca(CHNH)2] (3)

0.886 0.939

0.653 0.704

0.825 0.834

Na2[(CHNH)2Sr
Sr(CHNH)2] (4)

0.911

0.750

0.806

Na2[(CHNH)2Ba
Ba(CHNH)2] (5)

0.548

0.910

0.620

(CHNH)2Be
Be(CHNH)2 (6)

0.360

0.757

(CHNH)2Mg
Mg(CHNH)2 (7)

0.958

0.661

(CHNH)2Ca
Ca(CHNH)2 (8)

0.678

0.887

(CHNH)2Sr
Sr(CHNH)2 (9)

0.634

0.935

(CHNH)2Ba
Ba(CHNH)2 (10)

0.329

1.120

ΔEij measures the strength of the donor
acceptor interaction. Table 2 shows the substantial ΔEij values for all Na2[(CHNH)2Ae
Ae(CHNH)2] complexes, except for structure 3. The related donor
acceptor interaction is mainly from the lone pair (LP) of the Na atom to the Ae “empty” non-Lewis NBO orbital or to the antibonding N
Ae NBO orbital. These large ΔEij values reconfirm the strong stabilization effect of the Na atoms. The natural charges for the Ae atoms in the species 1
10 range from 0.34 to 1.12, which confirm the low oxidation state (þ1) of the central alkaline earth metals. The natural charges located on the R-diimine moieties range from
1.53 to
1.17 in 1
5 and from
1.12 to
0.66 in 6
10, indicating that the ligands behave as dianions in the former species and as monoanions in the latter compounds, a conclusion consistent with the bond length analysis. Energetics of the [Na]2[(CHNH)2Ae
Ae(CHNH)2] Complexes. In order to get further insights into the stability of the Ae
Ae bonds and the effect of the alkali metal ion, an energy analysis has been carried out. Two different dissociation routes are envisaged (Scheme 1). In route A, the optimized binuclear complexes Na2[(CHNH)2Ae
Ae(CHNH)2] dissociate into two mononuclear Na[Ae(CHNH)2] fragments, and the Ae
Ae bond dissociation energy (BDEA) may be evaluated by a simple subtraction. In route B, the dissociation proceeds in two steps. First, the alkali metals are eliminated from the whole dimeric compound, and the energy needed for this elimination reaction, DENa, is evaluated. Then, in the second step, the remaining binuclear complexes [(CHNH)2Ae
Ae(CHNH)2] further decompose into two mononuclear Ae(CHNH)2 fragments with a bond dissociation energy BDEB. The total energies of related binuclear complexes Na2[(CHNH)2Ae
Ae(CHNH)2] as well as the fragments (CHNH)2Ae
Ae(CHNH)2 and Ae(CHNH)2 Scheme 1. Two Possible Dissociation Pathways for the Na2[(CHNH)2Ae
Ae(CHNH)2] Complexes

Table 2. NBO Second-Order Interaction Energies of Na2[(CHNH)2Ae
Ae(CHNH)2] (1
5) species

donor NBO (i)

acceptor NBO (j)

ΔEij (kJ/mol)

Na2[(CHNH)2Be
Be(CHNH)2] (1)

LP*(1)Na

LP*(1)Be

26.6

Na2[(CHNH)2Mg
Mg(CHNH)2] (2)

LP*(1)Na LP*(1)Na

BD*(1) N
Mg BD*(1) N
Mg

16.6 16.7

Na2[(CHNH)2Ca
Ca(CHNH)2] (3)

Na2[(CHNH)2Sr
Sr(CHNH)2] (4)

Na2[(CHNH)2Ba
Ba(CHNH)2] (5)

LP*(1)Na

LP*(1)Ca

8.1

LP*(1)Na

BD*(1) N
Ca

4.8

LP*(1)Na

BD*(1) N
Ca

LP*(1)Na

LP*(1)Sr

55.9

LP*(1)Na

BD*(1) N
Sr

13.9

LP*(1)Na

BD*(1) N
Sr

13.9

LP*(1)Na LP*(1)Na

LP*(1)Ba BD*(1) N
Ba

171.5 38.4

LP*(1)Na

BD*(1) N
Ba

38.3

3116

4.9

dx.doi.org/10.1021/om200203n |Organometallics 2011, 30, 3113–3118

Organometallics

ARTICLE

Table 3. Total energies (in au) and Zero-Point Energies (in parentheses, kcal/mol) for [Na]2[(CHNH)2Ae
Ae(CHNH)2] and (CHNH)2Ae
Ae(CHNH)2 (structures 1 to 10) Predicted at the B3LYP and BP86 Levelsa structure 1

structure 2

structure 3

structure 4

structure 5

B3LYP


730.5758


1101.2370


2056.2516


762.4809


752.0239

BP86

(86.08)
730.5459

(82.21)
1101.2181

(81.06) 2056.3464

(80.30)
762.5430

(79.76)
752.1364

(79.81)

(78.54)

(77.80)

(77.47)

structure 7

structure 8

structure 9

(83.60) structure 6 B3LYP


405.8784


776.5470


1731.5458


437.7744


427.3138

(85.62)

(81.61)

(79.82)

(79.16)

(78.19)


405.8714


776.5576


1731.6709


437.8631


427.4513

(83.26)

(79.14)

(77.44)

(76.88)

(84.99)

Na[Be(CHNH)2]

Na[Mg(CHNH)2]

Na[Ca(CHNH)2]

Na[Sr(CHNH)2]

Na[Ba(CHNH)2]


365.2444


550.5861


1028.1061


381.2241


375.9969

(42.66)

(40.78)


365.2289


550.5767

(41.43)

(39.55)

BP86

B3LYP BP86

B3LYP BP86 a

(40.33)
1028.1546 (39.09)

(40.02)

(39.84)


381.2536


376.0520

(38.79)

(38.61)

Be(CHNH)2

Mg(CHNH)2

Ca(CHNH)2

Sr(CHNH)2

Ba(CHNH)2


202.9225


388.2454


865.7626


218.8794


213.6604

(41.78)

(40.44)

(39.18)

(38.81)

(38.68)


202.9106 (40.46)


388.2469 (39.27)


865.8233 (38.06)


218.9203 (37.67)


213.7264 (37.54)

Also included are Na[Ae(CHNH)2] and Ae(CHNH)2 species (Ae = Be, Mg, Ca, Sr, and Ba).

Table 4. Energy Analysis for Na2[(CHNH)2Ae
Ae(CHNH)2] Dissociation through Two Routes (Scheme 1) at the B3LYP and BP86 Levelsa route A

DE

ref

BP86/TZ2P

67.6

13

BP86/TZVPP

68.4

37

19.9

CpMg
MgCp

BP86/TZ2P

44.5

13

29.1

[C6H5
Be]2

BP86/TZVPP

39.9

37

72.6

34.6

39.8

64.7

39.4

CpCa
CaCp [C6H5
Be]2

BP86/TZ2P BP86/TZVPP

27.8 21.3

13 37

B3LYP

24.3

81.9

11.4

CpSr
SrCp

BP86/TZ2P

22.8

13

BP86 B3LYP

23.0 20.2

73.7 82.5

13.9 8.3

[C6H5
Be]2

BP86/TZVPP

16.7

37

CpBa
BaCp

BP86/TZ2P

16.1

13

BP86

22.2

76.6

12.5

B3LYP

18.8

84.3


5.2

BP86

20.1

90.9


10.9

B3LYP

53.9

77.4

BP86

54.6

73.8

B3LYP

40.0

BP86 Na2[(CHNH)2Ca
Ca(CHNH)2] Na2[(CHNH)2Sr
Sr(CHNH)2] Na2[(CHNH)2Ba
Ba(CHNH)2]

method

[C6H5
Be]2

BDEB

Na2[(CHNH)2Mg
Mg(CHNH)2]

compound CpBe
BeCp

DENa

Na2[(CHNH)2Be
Be(CHNH)2]

Table 5. Previously Predicted Dissociation Energy (DE, in kcal/mol) of Investigated Compounds Using [Ae
Cp] or [Ae
C6H5] as Fragments

route B

BDEA

compound

a

structure 10

method

Energy contributions in kcal/mol (including ZPVE corrections).

have been studied at the B3LYP and BP86 levels of theory, and the results are reported in Table 3 and Table 4. In Table 4, the zero-point-corrected values of BDEA for the dissociation of Na2[(CHNH)2Ae
Ae(CHNH)2] into two mononuclear Na[Ae(CHNH)2] fragments are positive, indicating that these reactions are endothermic, and the Na2[(CHNH)2Ae
Ae(CHNH)2] compounds are energy favorable. Thus it is anticipated that attempts to generate these alkaline earth metal
metal bonds stabilized by [Na
R-diimine]
may be successful. The BDEA values decrease as one descends the periodic table from Be to Ba. Compared with the previous theoretical work on

related systems (Table 5), this decreasing trend generally agrees with the theoretical results for CpAe
AeCp (BP86/TZ2P level) and [H5C6
Ae]2 (BP86/TZVPP level).37 The values for the homologous Ae
Ae bonds are similar to the diphenyl dialkaline earth metal(I) compounds and the binuclear metallocenes.13,37 As the theoretical energies for the elimination reactions of two Na atoms (DENa) are positive and are much larger than the bond dissociation energies for Ae
Ae bonding (BDEA), the dissociation route B is less favorable than route A (Scheme 1). The bond dissociation energies for (CHNH)2Ae
Ae(CHNH)2 (BDEB) may also be seen in Table 4. The values from Be to Sr are positive but smaller than the corresponding BDEA values, demonstrating that the [Na
R-diimine]
-stabilized Ae
Ae bonds are more stable than the isolated neutral (CHNH)2Ae
Ae(CHNH)2 species. These findings indicate that the [Na
R-diimine]
moiety 3117

dx.doi.org/10.1021/om200203n |Organometallics 2011, 30, 3113–3118

Organometallics may stabilize the Ae
Ae bonds not only via steric effects but also from electronic considerations, which are supported by the NBO analysis. The negative value of BDEB for (CHNH)2Ba
Ba(CHNH)2 indicates that the reaction is exothermic and the Ba
Ba bonding appears weak.

’ CONCLUSION Binuclear alkaline earth metal
metal bonds (from Be to Ba) are strongly stabilized by the anionic ligand [Na
R-diimine]
. The electronic structure of these five binuclear compounds (demonstrated by NBO analysis) clearly indicates covalent single bonding between metal
metal and ionic bonding between the alkali metal (Na) and the R-diimine ligand. Despite the shorter metal
metal bonds in (CHNH)2Be
Be(CHNH)2 and (CHNH)2Mg
Mg(CHNH)2, the stabilizing effects of the alkali metal on the Ae
Ae bonding is significant in view of the higher dissociation energies of the [Na]2[(CHNH)2Ae
Ae(CHNH)2] compounds compared to the (CHNH)2Ae
Ae(CHNH)2 species. ’ ASSOCIATED CONTENT

bS

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (20771103 and 20972169).

ARTICLE

(19) Yang, X.-J.; Yu, J.; Liu, Y.; Xie, Y.; Schaefer, H. F.; Liang, Y.; Wu, B. Chem. Commun. 2007, 2363–2365. (20) Yu, J.; Yang, X.-J.; Liu, Y.; Pu, Z.; Li, Q.-S.; Xie, Y.; Schaefer, H. F.; Wu, B. Organometallics 2008, 27, 5800–5805. (21) Liu, Y.; Li, S.; Yang, X.-J.; Yang, P.; Wu, B. J. Am. Chem. Soc. 2009, 131, 4210–4211. (22) Liu, Y.; Li, S.; Yang, X.-J.; Yang, P.; Gao, J.; Xia, Y.; Wu, B. Organometallics 2009, 28, 5270–5272. (23) Luo, Q.; Li, Q.-S.; Zhang, J.; Xie, Y.; Schleyer, P. v. R.; Schaefer, H. F. Inorg. Chem. 2006, 45, 6431–6434. (24) Wang, H.; Xie, Y.; Zhang, J. D.; King, R. B.; Schaefer, H. F. Inorg. Chem. 2007, 46, 1836–1846. (25) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (26) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (27) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100. (28) Perdew, J. P. Phys. Rev. B 1986, 33, 8822–8824. (29) We downloaded the basis sets from the internet address: https://bse.pnl.gov/bse/portal. (30) Kaupp, M.; Schleyer, P. v. R.; Stoll, H.; Preuss, H. J. Chem. Phys. 1991, 94, 1360–1366. (31) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639–5648. (32) Huzinaga, S. J. Chem. Phys. 1965, 42, 1293–1302. (33) Dunning, T. H. J. Chem. Phys. 1970, 53, 2823–2833. (34) Lee, T. J.; Schaefer, H. F. J. Chem. Phys. 1985, 83, 1784–1794. (35) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899–926. (36) Frisch, M. J. Gaussian 03 (E01); Gaussian Inc.: Wallingford, CT, 2004. (37) Krieck, S.; Yu, L.; Reiher, M.; Westerhausen, M. Eur. J. Inorg. Chem. 2010, 197–216. (38) Muresan, N.; Weyhermuller, T.; Wieghardt, K. Dalton Trans. 2007, 4390–4398.

’ REFERENCES (1) Resa, I.; Carmona, E.; Gutierrez-Puebla, E.; Monge, A. Science 2004, 305, 1136–1138. (2) Kress, J. W. J. Phys. Chem. A 2005, 109, 7757–7763. (3) Del Río, D.; Galindo, A.; Resa, I.; Carmona, E. Angew. Chem., Int. Ed. 2005, 44, 1244–1247. (4) Xie, Y.; Schaefer, H. F.; King, R. B. J. Am. Chem. Soc. 2005, 127, 2818–2819. (5) Xie, Z.-Z.; Fang, W.-H. Chem. Phys. Lett. 2005, 404, 212–216. (6) Richardson, S. L.; Baruah, T.; Pederson, M. R. Chem. Phys. Lett. 2005, 415, 141–145. (7) Philpott, M. R.; Kawazoe, Y. Chem. Phys. 2006, 327, 283–290. (8) Wang, Y.; Quillian, B.; Wei, P.; Wang, H.; Yang, X.-J.; Xie, Y.; King, R. B.; Schleyer, P. v. R.; Schaefer, H. F.; Robinson, G. H. J. Am. Chem. Soc. 2005, 127, 11944–11945. (9) Hanusa, T. P. Chem. Rev. 1993, 93, 1023–1036. (10) Westerhausen, M. Dalton Trans. 2006, 4755–4768. (11) Westerhausen, M.; G€artner, M.; Fischer, R.; Langer, J.; Yu, L.; Reiher, M. Chem.—Eur. J. 2007, 13, 6292–6306. (12) Stasch, A.; Jones, C. Dalton Trans. 2011, doi: 10.1039/ C0DT01831G. (13) Kan, Y.-H. J. Mol. Struct.: THEOCHEM 2009, 894, 88–92. (14) Xie, Y.; Schaefer, H. F.; Jemmis, E. D. Chem. Phys. Lett. 2005, 402, 414–421. (15) Li, Q. S.; Xu, Y. J. Phys. Chem. A 2006, 110, 11898–11902. (16) Green, S. P.; Jones, C.; Stasch, A. Science 2007, 318, 1754–1757. (17) Krieck, S.; G€orls, H.; Yu, L.; Reiher, M.; Westerhausen, M. J. Am. Chem. Soc. 2009, 131, 2977–2985. (18) Yang, P.; Yang, X.-J.; Yu, J.; Liu, Y.; Zhang, C.; Deng, Y.-H.; Wu, B. Dalton Trans. 2009, 5773–5779. 3118

dx.doi.org/10.1021/om200203n |Organometallics 2011, 30, 3113–3118