Metal tetrahydroborates and tetrahydroborato metalates. 30 [1] Solvates of alcoholato-, phenolato-, and bis(trimethylsilyl)amido-magnesium tetrahydroborates XMgBH4(Ln)

Metal Tetrahydroborates and Tetrahydroborato Metalates. 30 [1]

Solvates of Alcoholato-, Phenolato-, and Bis(trimethylsilyl)amido-Magnesium

Tetrahydroborates XMgBH₄(L_n)

Metall Tetrahydroborate und Tetrahydroborato Metallate, 30 [1].

Solvate der Alkoholato-, Phenolato- und Bis(trimethylsilyl)amino-magnesium-

tetrahydroborate XMgBH₄(L_n)

Mathias Bremer, Gerald Linti, Heinrich Nöth*, Martina Thomann-Albach and Gerhardt E. W. J. Wagner

Munich, Department of Chemistry, University of Munich

Received June 1st, 2004; revised October 28th, 2004.

Dedicated to Professor Martin Jansen on the Occasion of his 60^thBirthday

Abstract. A series of alcoholato and phenolato magnesium tetrahy-

droborates have been prepared as solvates (RO)Mg(BH₄)(OEt₂) by

1:1 reactions of ROH (R ϭ Me, Et, iPr, tBu, CH₂CMe₃, Et₂(tBu)C,

Ph₃C, Ph, 2,4,6-Me₃C₆H₂, (2,6-iPr)₂C₆H₃, (2,6-tBu₂)C₆H₃, 2-

(iPrO)C₆H₄, 2,6-(MeO)₂C₆H₃) with diethyl ether solutions of

EtMg(BH₄). Displacement of diethyl ether by thf, dme, and di-

glyme produced various solvates (RO)Mg(BH₄)(L)_n(L ϭ thf, dme,

diglyme, n ϭ 1, 2, 3). Most of these compounds are centrosymme-

tric dimers with a central Mg₂O₂four membered ring. Irrespective

of the composition, the ¹¹B NMR signals are 1:4:6:4:1 quintets

showing that the hydrogen atoms of the BH₄groups are magneti-

cally equivalent. The molecular structure of eight compounds

[(RO)Mg(BH₄)(OEt₂)]₂(R ϭ iPr, tBu, tBuCH_2,Et₃C, Me₃Si),

[(PhO)Mg(BH₄)(thf)₂]₂, (2,6-tBu₂C₆H₃O)Mg(BH₄)(thf)₂, (2,6-tBu_2-

C₆H₃O)-Mg(BH₄)(thf)(dme)) have been determined. Typical for all

compounds are hexacoordinated Mg atoms. The mononuclear

complexes feature bidentate BH₄groups while dinuclear species

show di- or tridentate BH₄groups depending on the number of

coordinated O atoms.

Mononuclear species are obtained only for compounds with bulky

R groups, e. g. 2,6-(tBu₂)(C₆H₃O)Mg(BH₄)(thf)₂with tridentate

BH₄groups. This compound has a linear O-C-Mg unit. In Me₂SO

the compound [(2-iPrO)C₆H₄O]Mg(BH₄)(OEt₂) underwent ligand

exchange to produce [(2-iPrO)C₆H₄O]₂Mg(OSMe₂)]₂.

Reaction of EtMgBH₄with a stoichiometric amount (1:1) of (Me_3-

Si)₂NH gives [((Me₃Si)₂N)-Mg(BH₄)(OEt₂)]₂, 46 with a tridentate

BH₄group. Its diethyl ether molecule can be replaced by thf, dme

and diglyme. Amongst these solvates the compound [(Me_3-

Si)₂N]Mg(BH₄)(diglyme) is mononuclear and its structure shows a

bidentate BH₄group. Compound 46 on repeated crystallization

from diethyl ether is transformed into [(Me₃SiO)Mg(BH₄)(OEt₂)]₂.

No compound with a BH₄group bonded to two Mg atoms has so

far been found.

Keywords: Boron; Magnesium; Tetrahydroborates; Alkoxo com-

pounds; BH₄bonding

Introduction

structure of non solvated Mg(BH₄)₂is still unknown but it

can be expected that the BH₄groups will bridge to at least

two Mg atoms forming either a chain like Be(BH₄)₂, or,

more likely, a layer or even an extended three dimensional

array in the solid state because the Mg atom prefers higher

co-ordination numbers than Be.

Solvates of magnesium tetrahydroborate, Mg(BH₄)₂(L)_n,

are, in general, mononuclear with bidentate BH₄units. Ty-

pical examples are Mg(BH₄)₂(thf)₃[6, 7], Mg(BH₄)₂(di-

glyme) [8] or Mg(BH₄)₂(py)₄[1]. Although a number of

“mixed” magnesium tetrahydroborates XMg(BH₄) are

known (X ϭ H [9], Cl [10, 11], Et [12Ϫ14]) there is no

information about their molecular state in solution nor on

their structures in the solid state. However, it is to be expec-

ted that the bonding mode for the BH₄group may change

within the series of magnesium tetrahydroborates

XMgBH₄(L)_n(X ϭ RO, R₂N; L ϭ various ethers, n ϭ 1,

2, 3), and that the substituents X will bridge Mg atoms

more readily than BH₄, generating di-, tri- or even polynu-

Beryllium and magnesium tetrahydroborate bridge the gap

between the ionic metal tetrahydroborates such as NaBH₄

or KBH₄and the covalent metal tetrahydroborates exempli-

fied by Al(BH₄)₃and Zr(BH₄)₄. While the structure of

Be(BH₄)₂is well established for the solid state [2] showing

hexacoordinated Be atoms in a helical chain, the analyses

of its vibrational spectra allows no firm conclusion as to

the structure in solution, even using isotopic labeling [3, 4].

Calculations for the monomeric molecule shows [5] that the

most stable species has D_3dsymmetry. In contrast, the

* Prof. Dr. Dr. h.c. H. Nöth

Department of Chemistry, University

Butenandstr. 5Ϫ13

D-81377 München

E-Mail: H.Noeth@lrz.uni-muenchen.de

Z. Anorg. Allg. Chem. 2005, 631, 683Ϫ697

DOI: 10.1002/zaac.200400420

683

M. Bremer, G. Linti, H. Nöth, M. Thomann-Albach, G. E. W. J. Wagner

clear species, and the degree of association will also depend

on the type and number of coordinated solvent molecules

L. These aspects initiated the present study.

Table 1 Solubilities of various (RO)MgBH₄compounds in ethers

(mol/l) at 20 °C

RO

Et₂O

thf

dme

diglyme

a,b)

a)

MeO

EtO

iPrO

tBuO

Et₃CO

Me₃CCH₂O

Ph₃CO

0.48

0.11

0.04

0.11

0.09

0.56

0.03

0.07

0.35

0.32

0.14

0.12

0.01

0.11

0.14

0.37

0.12

Ϫ

0.18

Ϫ

0.27

0.08

Ϫ

Solvates of Organyloxomagnesium Tetrahydroborates

0.04

0.43

0.02

0.87

Ϫ

0.63

Ϫ

a)

e)

Synthesis

a)

Mixed magnesium tetrahydroborates have been prepared by

several routes. These are summarized by equations (1) to

(5) ignoring formation of solvates:

Ϫ

0.02

Ϫ

f)

PhO

0.17

0.31

0.77

0.36

Ϫ

c)

2,4,6-Me₃C₆H₂O

2,6-iPr₂C₆H₃O

2,6-tBu₂C₆H₃O

2-(iPrO)C₆H₄O

2,6-(MeO)₂C₆H₃O

0.7

d)

g)

0.14

Ϫ

>0.32

0.06

MgH₂ϩ BH₃·thf Ǟ HMg(BH₄) ϩ thf

ClMgH ϩ BH₃·thf Ǟ ClMg(BH₄) ϩ thf

(1)

(2)

(3)

0.05

0.08

0.13

^a)at 5 Ϫ 8 °C; ^b)1:1 mixture of Et₂O and thf; ^c)at Ϫ20 °C; ^d)crystals separa-

ted from a 1M solution at Ϫ20 °C; ^e)2:1 diethyl ether diglyme solution;

^f)solubility after heating to reflux and cooling to 20 °C; ^g)a 3.5 : 1 diethyl

ether/dme solution was used.

XMgR ϩ 2 BH₃·thf Ǟ XMg(BH₄) ϩ RBH₂ϩ thf

(X ϭ Cl, Br; R ϭ Et, Pr, Ph)

RMgCl ϩ M(BH₄) Ǟ RMg(BH₄) ϩ MCl

(R ϭ Me, Et; M ϭ Li, Na)

(4)

(5)

action of diethylketone with (tBu)Mg(BH₄), which leads to

(tBuEt₂CO)Mg(BH₄)(OEt₂), 15, as shown in equation (7).

This demonstrates that the MgϪC bond is more reactive

than the BH bonds of the MgBH₄group. The alkoholato

and phenolato magnesium tetrahydroborates are much less

soluble in diethyl ether than EtMg(BH₄) but most of them

are sufficiently soluble in benzene for NMR studies. The

solubility has been determined for 13 compounds in four

different ethers (see Table 1).

It is surprising that (MeO)Mg(BH₄) is much better

soluble in diethyl ether than (iPrO)Mg(BH₄) or

(tBuO)Mg(BH₄) in contrast to the solubility in thf and dme.

Several of the dme and diglyme solvates are only sparingly

soluble in their own ether, but all dissolve readily in Me₂SO.

¹¹B NMR studies in acetonitrile show that at ambient tem-

perature acetonitrile is not reduced rapidly by these magne-

sium tetrahydridoborates as the quintet for the BH₄group

can be observed for several hours.

EtMg(BH₄) ϩ HX Ǟ XMg(BH₄) ϩ EtH

(X ϭ Cl, RO)

(RO)Mg(BH₄)(L_n) ϩ m LЈ Ǟ (RO)Mg(BH₄)(LЈ)_mϩ n L

EtMgBH₄ϩ tBu₂CϭO Ǟ (tBu₂EtCO)Mg(BH₄)

(6)

(7)

EtMgBH₄was the first in the series of mixed magnesium

tetrahydroborates obtained by reaction (4), and both,

LiBH₄and NaBH₄were used for the metathesis [12, 13].

HMg(BH₄) resulted by allowing a large excess of solid

MgH₂to react with BH₃·thf in thf solution [9]. ClMg(BH₄)

was synthesized from HMgCl or EtMgCl and BH₃·thf in

thf [10, 11]. For the preparation of organyloxo and amido

magnesium tetrahydroborates we chose reactions according

to equation (5) because other starting materials such as

RMgX and XMgCl (X ϭ RO, R₂N) are in most cases not

well defined. Several reactions according to equation (5)

were followed by ¹¹B NMR spectroscopy. These studies

showed that the BϪH bond of the BH₄group of

EtMg(BH₄) is not attacked by the protic reagents ROH and

R₂NH in contrast to the MgϪC bond.

Table

2 lists the isolated ether solvates of the

(RO)Mg(BH₄) compounds, their ¹H and ¹¹B chemical shifts

(in ppm) and the J(¹¹B¹H) coupling constants in Hz. Also

1

¹J(¹⁰B¹H) values are quoted when observed.

Diethyl ether solvates (RO)Mg(BH₄)(OEt₂)_n( n ϭ 1,

compounds 1 to 11, 14; n ϭ 2, compounds 12, 13) were

readily obtained in 1: 1 reactions and in good yield from

diethyl ether solutions of EtMg(BH₄) and ROH. These diet-

hyl ether solvates on treatment with thf, dme or diglyme

provided the thf solvates 16 to 25, the dme solvates 26 to

34, and the diglyme solvates 35 to 44 as shown in equation

(6). Ether displacement is achieved according to the series

Et₂O < thf < dme < diglyme < Me₂SO. Usually, the ether

displacement is quantitative, but in case of thf displacement

by dme a mixed solvate was obtained in case of (2,6-tBu_2-

C₆H₃O)Mg(BH₄) which was isolated as (2,6-tBu_2-

C₆H₃O)Mg(BH₄)(thf)(dme), 33. An alternative route to al-

coholato magnesium tetrahydroborates was found in the re-

Most (RO)Mg(BH₄) compounds bind one diethyl ether

molecule, exceptions being the compounds 12 and 13 which

coordinate two diethyl ether molecules. This suggests, that

the structures of the latter two differ from the 1 : 1 solvates.

Most likely 12 and 13 are mononuclear species while the

others are dinuclear (v. i.).

The base thf coordinates more strongly than Et₂O to the

Mg centers of (RO)Mg(BH₄). Most compounds contain

two thf solvate molecules (16, 28, 23, and 24) but even three

thf molecules may act as ligands as shown for compound

22 which most likely is mononuclear. A 2 : 3 ratio of Mg :

thf was observed for 17. Solvates of (RO)Mg(BH₄) with one

dme molecule (26, 27, 33) may be mononuclear, while those

of formula (RO)Mg(BH₄)(dme)_0.5(28, 29) are probably di-

684

zaac.wiley-vch.de

Z. Anorg. Allg. Chem. 2005, 631, 683Ϫ697

Metal Tetrahydroborates and Tetrahydroborato Metalates

Table 2 Composition of the isolated solid (RO)Mg(BH₄)(L)_ncompounds, numbering of the compounds in bold letters, ¹¹B (upper line)

1

chemical shifts (in ppm), and the corresponding coupling constants J(¹¹B¹H) (Hz, in parenthesis) of the ¹¹BH₄unit. (in addition for 8,

1

24, 25, 34, 44: J(¹⁰B¹H)

Et₂O

thf,n

dme,n

diglyme,n

MeO

1, n ϭ 1

Ϫ44.2 (78.6)

Ϫ

16, n ϭ 2,

Ϫ2.0 (81.1)

0.42 (81.6)...

17, n ϭ 1.5,

Ϫ38.5 (79.1)

Ϫ

35, n ϭ1,

Ϫ3.4 (80.2)

Ϫ

a

a)

EtO

2, n ϭ 1

26, n ϭ1,

Ϫ41.2 (79.6)

36, n ϭ 1,

Ϫ42.3 (br)

Ϫ

a)

b)

a)

c)

a)

Ϫ42.6 (81.0)

Ϫ0.33 (81.5)

3, n ϭ 1,

Ϫ43.7 (82.6)

0.40 (84.4)

4, n ϭ 1,

Ϫ43.0 (82.1)

0.55 (82.4)

5, n ϭ 1,

Ϫ42.5 (80.9)

0.75 (81.0)

6, n ϭ1,

Ϫ44.4 (82.8)

0.48 (82.8)

7, n ϭ 1,

Ϫ40.9 (81.2)

Ϫ0.16 (82.8)

8, n ϭ1,

a)

0.42 (80.1)

iPrO

18, n ϭ 1,

Ϫ42.1 (81.6)

Ϫ

19, n ϭ 1,

Ϫ42.7 (81.4)

0.61 (83.4)

20, n ϭ 1,

Ϫ43.0 (82.8)

0.55 (82.3)

Ϫ

27, n ϭ 1,

Ϫ41.3.(82.3)

Ϫ

28, n ϭ 0.5,

Ϫ40.3 (79.1)

Ϫ

29, n ϭ 0.5,

Ϫ42.9 (82.0)

0.55 (81.9)

Ϫ

37, n ϭ 2,

Ϫ41.4 (82.3)

Ϫ

38, n ϭ 0.33,

Ϫ42.9 (82.3)

a)

c)

a)

tBuO

a) d)

a)

Et₃CO

Me₃CCH₂O

Ph₃CO

MeOCH₂CH₂O

PhO

39, n ϭ 0.5,

Ϫ43.7 (82.4)

Ϫ

40, n ϭ 1.33,

Ϫ35.7 (80.9)

Ϫ0.29 (81.5)

41, n ϭ 1,

Ϫ42.8 (83.4)

0.25 (82.3)

Ϫ

a)

Ϫ

a)

30, n ϭ 0.66,

Ϫ41.5 (80.3)

c)

Ϫ35.9 (81.6)

0.42 (81.5, 27.4)

9, n ϭ1,

Ϫ40.9 (81.2)

21, n ϭ 2

Ϫ41.4 (79.5)

0.72 (83.1)

31, n ϭ 1.5,

Ϫ39.4 (84)

Ϫ0.80 (79.1)

Ϫ

<e

a)

Ϫ

2,4,6-Me₃C₆H₂O

2,6-iPr₂C₆H₃O

2,6-tBu₂C₆H₃O

2-(iPrO)C₆H₄O

10, n ϭ 1,

Ϫ42.4 (81.3)

Ϫ0.29 (81.5, 27.3)

11, n ϭ1,

Ϫ40.8 (97.5)

Ϫ

c)

b)

22, n ϭ 3

32, n ϭ 1,

Ϫ40.4 (80.8)

Ϫ0.27 (81.5)

33, n ϭ 1, 1 thf,

Ϫ41.5 (82.2)

42, n ϭ1,

Ϫ43.3 (80.7)

0.37 (81.9)

43, n ϭ1,

Ϫ42.6 (82.2)

0.45 (82.0)

Ϫ

c) e)

a)

b)

a)

Ϫ41.8(82.6)

0.36 (83.3)

23, n ϭ 2,

12, n ϭ2,

Ϫ41.0 (79.1)

a)

Ϫ42.6 (79.6)

Ϫ

13, n ϭ 2,

Ϫ41.7 (br)

(81.5), -0.12 (81.5)

24, n ϭ 2,

a) f)

c)

Ϫ0.09

Ϫ38.5 and Ϫ42.6

broad, nresolv.

Ϫ.32 (81.6, 27.3)

25, n ϭ 1

Ϫ36.0 (81.5)

Ϫ0.31 (82.1, 27.4)

b)

(2,6-MeO)₂C₆H₃O

14, n ϭ 1

Ϫ45.2 (81.4)

0.31 (81.5 )

34, n ϭ 1.5

Ϫ34.5 (81.0)

Ϫ0.31 (81.6, 27.1)

44, n ϭ 0.33

Ϫ34.6 (br)

Ϫ0.31 (81.5, 27.3)

a) g)

b)

Et₂(tBu)CO

15, n ϭ 1

Ϫ43.0 (82.1)

Ϫ

a)

Ϫ0.4 (82.2)

^a)in C₆D₆, ^b)in d₆-Me₂SO, ^c)in thf, ^d)in Et₂O, ^e)in Me₂SO: Ϫ35.5 ppm (81.6), ^f)in Me₂SO Ϫ36.0 ppm (80.9), ^g)in Me₂SO: Ϫ35.9 (81.4)

nuclear with a bridging dme ligand between two Mg atoms.

Compound [(PhO)Mg(BH₄)]₂(dme)₃, 32, must surely be di-

nuclear. Provided that the Mg atoms are equivalent, one

can speculate that one dme molecule bridges the Mg atoms,

while the other two are bonded as monodentate dme li-

gands. This would require bidentate BH₄groups (v. i.). On

the other hand, we assume that compounds of type

(RO)Mg(BH₄)(diglyme) (36 and 41) are mononuclear with

hexacoordinated Mg atoms and bidentate BH₄groups,

while that of composition (RO)Mg(BH₄)(diglyme)_0.33, 38,

most likely binds one Mg atom per solvent oxygen atom. If

we assume that all compounds contain a hexacoordinated

Mg atom then the structures A to E shown in scheme 1 are

feasible. Only one isomer is shown.

While no ligand exchange to produce Mg(BH₄)₂and

Mg(OR) has been observed during the ether displacement

reactions this happened during the crystallization of [(2-

iPrO)C₆H₄O]-Mg(BH₄)(OEt₂) from Me₂SO as shown in

Eq. (8).

2 [(2-iPrO)C₆H₄O]Mg(BH₄)(OEt₂) ϩ 7 Me₂SO Ǟ

[(2-iPrO)C₆H₄O]₂Mg(OSMe₂) ϩ [Mg(OSMe₂)₆](BH₄)₂

(8)

Z. Anorg. Allg. Chem. 2005, 631, 683Ϫ697

zaac.wiley-vch.de

685

M. Bremer, G. Linti, H. Nöth, M. Thomann-Albach, G. E. W. J. Wagner

Scheme 1

Figure 2 Molecular structure of dimeric (tBuO)Mg(BH₄)(OEt₂),

(4).

˚

Selected bond lengths/A: MgϪO1 1.947(2), MgϪO2 2.045(2),

MgϪO1A 1.949(2), O1ϪC1 1.436(4), O2ϪC7 1.455(4), O2ϪC5

1.453(4), MgϪH1 2.05(4), MgϪH2 2.02(4), MgϪH3 2.11(4),

BϪH1 1.19(4), BϪH2 1.20(4), BϪH3 1.10(4), BϪH4 1.10(4),

Mg···B 2.268(5). Ϫ Selected bond angles/°: O1ϪMgϪO2 107.5(1),

BϪMgϪO1 122.5(1), BϪMgϪO2 108.6(1), BϪMgϪO1A 123.4(1),

O2ϪMgϪO1A 108.0(1), O1ϪMgϪO1A 84.0(1), MgϪO1ϪC1

132.7(2), MgϪO1ϪMgA 96.0(1), MgAϪO1ϪC1 131.3(2),

MgϪO2ϪC5 19.7(2), MgϪO2ϪC7 124.8(2), C5ϪO2ϪC7 114.3(3),

H1ϪMgϪH2 53(1), H1ϪMgϪH3 51(1), H2ϪMgϪH3 53(1),

H1ϪBϪH2 99(2), H1ϪBϪH3 103(2), H1ϪBϪH4 122(3),

H2ϪBϪH3 107(3), H2ϪBϪH4 123(3), H3ϪBϪH4 111(3).

Figure 1 Molecular structure of dimeric (iPrO)Mg(BH₄)(OEt₂), (3).

Selected bond lengths/A: MgϪO1 1.945(2), MgϪO1A 1Ϫ950(2),

˚

MgϪO2 2.022(2), Mg··B 2.251(3), MgϪH10 2.08(3), MgϪH11

2.06(3), BϪH10 1.11(3), BϪH11 1.16(3), BϪH12 1.11(3). BϪH13

pounds. While the lengths of the Mg-O bonds are practi-

cally the same for (iPrO)Mg(BH₄)(OEt₂), 3, (tBuO)Mg(B-

H₄)(OEt₂), 4, and (tBuCH₂O)Mg(BH₄)(OEt₂), 6, those of

(Et₃CO)Mg(BH₄)(thf), 20, are significantly longer. This is

obviously a consequence of the high steric demand of the

Et₃C group. The diethyl ether molecule is most firmly atta-

ched to the Mg centre in the neopentyloxy derivative 6. The

difference in the MgϪOR and MgϪOEt₂bond lengths is

largest in 4. Moreover, the BϪMgϪO1 bond angle is shar-

per by 6° in compound 6 than in the other three com-

pounds. The smaller the BϪMgϪO1 bond angle the larger

the BϪMgϪO2 bond angle.

1.06(3).

Ϫ Selected bond angles/°: O1ϪMgϪO1A 84.0(1),

MgϪO1ϪMg1A 96.0(1), MgϪH10ϪB 84(2), MgϪH11ϪB 83.(2),

MgϪBϪH13 177(2), H10ϪBϪH11 107(2), H10ϪBϪH12 102(2),

H10ϪBϪH13 115(2), H11ϪBϪH12 108(2), H11ϪBϪH13 111(2),

H12ϪBϪH13 113(2).

Molecular Structures

Knowledge of the structure of the (RO)Mg(BH₄)(L)_ncom-

pounds could be helpful for the interpretation of NMR and

IR spectroscopic data. For this reason we determined the

structures of several of the organyloxo magnesium tetrahy-

droborates.

Ϫ

If we consider the BH₄anion as a “pseudohalide” (BH₄

has the same number of valence electrons as fluoride, but

its effective ions radius corresponds with that of the bro-

mide ion), then the Mg atoms in [(RO)Mg(BH₄)(OEt₂)]₂are

tetracoordinated. But the bond angles around the Mg atom

are strongly distorted from a tetrahedral geometry, conside-

ring only the MgO₃B core, due to the sharp O1-Mg1-O1A

bond angle of the Mg₂O₂ring. However, the Mg atoms in

these compounds can also be looked at as being hexacoor-

dinated, if we take the MgϪH interactions into account.

Then the coordination polyhedron around the Mg atom is

a strongly distorted octahedron. This is, in particular, due

to the very sharp HϪMgϪH bite angles which are close

to 70°.

Compounds (RO)Mg(BH₄)(OEt₂) (R ϭ Me₂CH, 3;

Me₃C, 4; Me₃CCH₂, 6, and Me₃Si, 50) crystallize in the

monoclinic system, space group P2₁/n. They all are dinu-

clear with the RO oxygen atom bridging the two Mg atoms.

The molecules feature planar four-membered Mg₂O₂rings

with a crystallographic inversion centre in the middle of this

ring. A consequence of the inversion centre is that the li-

gands and substituents are trans oriented. Their structures

including the triclinic 5 are shown in Figures 1 to 5.

Typical for these compounds are tridentate BH₄groups.

˚

The Mg···B distances vary from 2.168(5) to 2.244(6) A. The

shortest distance is observed for the neopentyl derivative 6.

Moreover, also the B-Mg-O angle of this compound is quite

different compared with those of the other four com-

In this context we include here also the molecular struc-

ture of [(Me₃SiO)Mg(BH₄)(OEt₂)]₂, 50 (v. i., see Figure 5).

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Metal Tetrahydroborates and Tetrahydroborato Metalates

Figure 3 Molecular structure of dimeric (Et₃CO)Mg(BH₄)(thf),

(20).

Selected bond lengths/A: MgϪO1 1.963(2), MgϪO2 2.034(2),

Figure 4 Molecular structure of dimeric (Me₃CCH₂O)Mg(BH₄)-

˚

(OEt₂), (6), in the solid state. Selected bond lengths/A: MgϪO1

1.942(3), MgϪO2 2.028(3), MgϪO1A 1.949(3), Mg^..B 2.244(6).

MgϪO1A 1.960(2), Mg···B 2.290(5), O1ϪC1 1.437(3), O2ϪC8

1.441(5). O2ϪC11 1.44(5), MgϪH1 2.12(4), MgϪH2 2.18(4),

MgϪH3 2.09(4), BϪH1 1.13(4), BϪH2 1.10(3), BϪH3 1.14(4),

BϪH4 0.97(4). Ϫ Selected bond angles/°: O1ϪMgϪO2 108.5(1),

O1ϪMgϪO1A 84.8(1), O1ϪMgϪB 123.4(1), O2ϪMgϪB 104.8(1),

BϪMgϪO1A 125.2(1), MgϪO1ϪC1 132.5(2), MgϪO1ϪMgA

95.2(1), MgAϪO1ϪC1 132.3(2), MgϪO2ϪC11 127.0(2),

MgϪO2ϪC8 123.7(2), C8ϪO2ϪC11 109.1(3), H1ϪMgϪH2 49(1),

H1ϪMgϪH2 51(1), H2ϪMgϪH3 50(1), MgϪH1ϪB 84(1),

MgϪH2ϪB 82(2), H3ϪMgϪB 85(2), H1ϪBϪH2 95.2(2),

H1ϪBϪH3 106.5(26), H1ϪBϪH4113(3), H2ϪBϪ3 107(2),

H2ϪBϪH4 108(3), H3ϪBϪH4 116(3).

MgϪHa 2.24, MgϪHb 2.15, MgϪHc 2.15, BϪHa 1.11, BϪHb

0.91, BϪHc 1.11, BϪHd 1.15.

O1ϪMgϪO2 112.0(1), O1ϪMgϪO1a 84.4(1), BϪMgϪO1

117.5(2), BϪMgϪO2 110.5(2), BϪMgϪO1a 126.8(2),

Ϫ Selected bond angles/°:

O1ϪMgϪHa 98.0(2), O2ϪMgϪHb 101.5(2), O1ϪMgϪHc

146.0(2), HaϪMgϪHb 40.4(1), HϪMgϪHc 49.3(1), HbϪMgϪHc

49.1(1), HaϪBϪHb 97.1(7), HaϪBϪHc 111.7(6), HbϪBϪHc

124.5(8), HaϪBϪHd 99.1(6), HbϪBϪHd 104.1(7), HcϪBϪHd

116.0(7).

˚

Its Si-O bond is comparatively short (1.612(2) A), and the

oxygen atom is located in a planar environment. The short

MgϪB distance fits to a tridentate BH₄group as found for

compounds 3 to 6.

The phenolate (PhO)Mg(BH₄)(thf)₂, 21, may be mono-

nuclear with a tridentate BH₄anion and hexacoordinated

Mg or dinuclear with a bidentate BH₄group. The latter

alternative is realized as shown by its molecular structure

(Figure 6).

This molecule has also a crystallographic centre of sym-

metry. The MgϪO bonds are somewhat longer than in the

Figure

(OEt₂), (50).

5 Molecular structure of dimeric (Me₃SiO)Mg(BH₄)-

˚

previous examples (by 0.02 A) but the Mg-O bond to one

˚

Selected bond lengths/A: MgϪO1 2.015(3), MgϪO2 1.960(2),

MgϪO2A 1.964(2), MgϪH1 2.07(4), MgϪH2 2.00(4). MgϪH3

2.22(4), O2ϪSi2 1.621(2), Mg···B1 2.254(5), BϪH1 1.06(4), BϪH2

1.12(4), BϪH3 1.09(4), BϪH4 1.09(4). Selected bond angles/°:

O2ϪMgϪO2A 86.5(1), MgϪO2ϪMgA 93.5(1), O2ϪMgϪO1

108.4(1), MgϪBϪH4 177(2), H1ϪBϪH4 114(3), H4ϪBϪH2

115(3), H4ϪBϪH3 108(3), H1ϪBϪH2 110(3), H1ϪBϪH3 100(3),

H2Ϫ BϪH3 108(3).

˚

of the thf molecules is shorter (1.984(3) A) than the MgϪO

˚

bond to the second thf molecule (2.074(3) A). Because the

MgϪOϪMg bond angle is larger than in the diethyl ether

complexes 3 to 6, the Mg···Mg distance across the four

˚

membered ring has enlarged to 3.095 A, corresponding to a

˚

difference of 0.2 A. The BH₄group is present as a bidentate

ligand, and, as a consequence, the MgϪB distance is longer

than in the diethyl ether complexes with tridentate BH₄

groups. It is well known that the M···B distances decrease

in the order monodentate > bidentate > tridentate simply

for geometric reasons. Thus, as Edelstein [15] has pointed

out, the M-B distance is a good criterion for the type of

bonding of the BH₄group in mononuclear metal tetrahy-

droborates.

In contrast to the dinuclear structure of 21 the compound

(2,6-tBu₂C₆H₃O)Mg(BH₄)(dme)(thf), 33, is mononuclear.

Its molecular structure is shown in Fig. 7. The Mg atom is

hexacoordinated. As expected, the coordination sphere is a

Z. Anorg. Allg. Chem. 2005, 631, 683Ϫ697

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687

M. Bremer, G. Linti, H. Nöth, M. Thomann-Albach, G. E. W. J. Wagner

Figure 7 Molecular structure of (2,6ϪtBu₂C₆H₃O)Mg(BH₄)(thf)-

(dme), (33).

˚

Selected bond lengths/A: MgϪO1 1.883(3), MgϪO2 2.094(4),

MgϪO3 2.112(4), MgϪO4 2.107(4), Mg···B 2.446(6), O1ϪC1

1.335(4), O2ϪC15 1.453(4), O2ϪC18 1.451(6), O3ϪC20 1.435(5),

O3ϪC21 1.429(6), O4ϪC19 1.443(5), O4ϪC22 1.429(6). Ϫ Selec-

ted bond angles/°: O1ϪMgϪO2 95.3(1), O1ϪMgϪO3 96.5(1),

O1ϪMgϪO4 121.9(1), O2ϪMgϪO3 163.2(1), O2ϪMgϪO4

88.5(1), O3ϪMgϪO4 75.1(1), BϪMgϪO1 126.8(1), BϪMgϪO2

94.1(2), BϪMgϪO3 96.5(1), BϪMgϪO4 110.5(1), MgϪO1ϪC1

177.0(2), MgϪO2ϪC18 125Ϫ4(3), MgϪO2ϪC15 127.9(2),

C15ϪO2ϪC18 106.3(3), MgϪO3ϪC20 112.0(3), MgϪO3ϪC21

123.5(3), C20ϪO3ϪC21 113.4(3), MgϪO4ϪC19 117.4(3),

MgϪO4ϪC22 131.0(3), C19ϪO4ϪC22 110.7(3).

Figure 6 Molecular structure of dimeric (PhO)Mg(BH₄)(thf)₂, (21),

in the solid state.

Selected bond lengths/A: MgϪO1 1.984(3), MgϪO2 2.094(3).

˚

MgϪO3 2.074(3), MgϪO1A 2.037(3), Mg···B 2.454(5), O1ϪC1

1.360(4). Selected bond angles/in °: BϪMgϪO1 122.2(2),

BϪMgO2 93.6(1), BϪMgϪO3 121.7(2), O1ϪMgϪO2 93.3(1),

O1ϪMgϪO3 116.1(3), O2ϪMgϪO3 83.4(1), O1ϪMgϪO1A

79.4(1), O2ϪMgϪO1A 165.0(1), MgϪO1ϪMgA 100.6(1),

MgϪO1ϪC1 131.7(2), O3ϪMgϪO1A 88. (1), C7ϪO2ϪC10

109.0(3), C11ϪO3ϪC14 108.5(3).

highly distorted octahedron due to the sharp OϪMgϪO

and HϪMgϪH bond angles. The sum of bond angles about

the “trigonal plane” (pentacoordinated Mg with B and 4 O

atoms) is 359.2°, the angle BϪMgϪO1 being most open

with 126.8°, while the angle BϪMgϪO2 is smallest

(110.5°). All other angles are close to 90° with maximal

deviations of 6.5 and 15°.

d₆-Me₂SO or the coresponding ether solution. Data recor-

ded in C₆D₆solution do not deviate much from those obtai-

ned in the ether solutions, and this holds also for the cou-

1

pling constant J(¹¹B¹H). We take this as evidence that the

molecular state of the compounds studied is the same in

nonpolar and polar solvents. Usually well resolved 1 : 4 : 6

: 4 : 1 quintets were observed in the ¹¹B NMR spectra as

expected for the presence of BH₄groups. Even at low tem-

perature (Ϫ60 °C), no change in the coupling pattern was

observed. Therefore, the NMR method gives no indication

how the BH₄groups binds to the Mg atom, i. e. via one,

two, or three of its hydrogen atoms because all four hydro-

gen atoms of the BH₄groups are magnetically equivalent.

This is typical for most metal tetrahydridoborates [16, 17,

18]. The shielding of the boron nuclei as well as the cou-

pling constant change only little by changing the ether sol-

vent. For the diethyl ether solvates of the whole range of

RO groups, δ¹¹B varies only between Ϫ40.8 and

Ϫ44.2 ppm, for thf solutions from Ϫ38.5 to Ϫ42.7 ppm, for

dme solutions from Ϫ40.3 to Ϫ42.9 ppm and for diglyme

solutions from Ϫ41.6 to Ϫ43.7 ppm. This suggests that the

boron atoms in all of these compounds carry more or less

the same charge. The δ¹¹B values are close to those of the

alkali metal tetrahydroborates [19] and amine solvates of

Mg(BH₄)₂[1]. In contrast to the ether solvates the ¹¹B nu-

clei are less well shielded in d₆-Me₂SO as a solvent. As was

shown for Mg(BH₄)₂ether complexes these are present in

d₆-Me₂SO solution as the ionic compound [Mg(OS-

Me₂)₆](BH₄)₂(OSMe₂)₂, whose structure has been determi-

A striking feature of the structure of 33 is the almost

linear C1ϪO1ϪMg1 unit of the phenolate ligand

(177.0(2)°). This is in accord with a very short C1ϪO1

˚

(1.335(4) A) and MgϪO1 bond (1.883(3) A). The CϪO

bond lengths suggests a bond order of 1.5. Its MgϪO bond

˚

is 0.2 A shorter than all other MgϪO bonds observed in

this study. This suggests multiple bond contributions (the

shrinking of the MgϪO distance is due only to the dicoor-

dinated O atom because the Mg atom remains hexacoordi-

nated). In accord with a bidentate BH₄group is the com-

˚

paratively long Mg···B distance (2.446 A).

It follows from the structural data now available that all

(RO)Mg(BH₄)(L)_nmolecules have a hexacoordinated Mg

atom. Except for compounds with sterically demanding

phenolate groups the RO group form bridges between the

Mg atoms. The remaining coordination sites are either oc-

cupied by O atoms of the ether ligands or H atoms of the

BH₄anions which may be either bidentate or tridentate.

¹¹B NMR Spectra

1

¹¹B chemical shift data and J(¹¹B¹H) coupling constants

of the (RO)Mg(BH₄) compounds were recorded in C₆D₆or

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Metal Tetrahydroborates and Tetrahydroborato Metalates

Scheme 3

Scheme 2

ned in the solid state [1]. δ¹¹B in d₆-Me₂SO solution for

all compounds (RO)Mg(BH₄) is found in a narrow range

(Ϫ35( 1) ppm. We rationalize this by the formation of free

BH₄anions as indicated in equation (9). Thus, ¹¹B NMR

data for the (RO)Mg(BH₄) compounds in deutero benzene

solutions not only point to an interaction of the BH₄units

with the Mg atoms as determined for the solid state but

also to a dependence from the coordinated ether.

distinguish between bidentate- and tridentate-bonded BH₄

groups for mononuclear and dinuclear (RO)Mg(BH₄)(L_n)

compounds. The IR spectra, however, often did not show a

good enough resolution for the BH stretching vibrations in

nujol mulls. Although the number of resolved bands differ

for the series of compounds (RO)Mg(BH₄)(OEt₂) the ob-

served pattern corresponds best with tridentate BH₄groups

(local C_3v-symmetry). There is a band at high wave numbers

(2472 to 2426 cm^Ϫ1, ν (BH_terminal) in addition to two bands

at lower frequencies (range 2200 to 2180 cm^Ϫ1, ν_asand

ν_symBH₃). The IR spectra of the thf solvates show also very

different resolutions of the bands in the BH stretching re-

gion. With the exception of (tBuO)Mg(BH₄)(thf), 19, which

shows an IR spectrum typical for a tridentate BH₄group,

all others exhibit intense bands at about 2380, 2290 to 2250

and 2200 to 2240 cm^Ϫ1. Two sets of two bands (ν_asand

ν_symm) are expected for bridging and terminal ¹¹BH₂units.

The observed patterns correspond closely to those for a bi-

dentate BH₄group. Compound [(2-iPrO)C₆H₄O]-

Mg(BH₄)(thf)₂may have the structure of either F or G.

[(2,6-MeO)₂C₆H₃O]Mg(BH₄)(thf), 25, should have struc-

ture H because the methoxy groups are not equivalent (by

NMR) which points to the presence of a coordinated and

a non-coordinated MeO group.

(RO)Mg(BH₄)(L)_nϩ 5 Me₂SO Ǟ

Ϫ

[(RO)Mg(OSMe₂)₅]^ϩϩ BH₄n L (9)

2 (RO)Mg(BH₄)(L)_nϩ 7 Me₂SO Ǟ

(RO)₂Mg(OSMe₂) ϩ [Mg(OSMe₂)₆] (BH₄)₂ϩ n L (10)

For several (RO)Mg(BH₄) compounds in ether solutions

the proton signals for the BH₄group was found in the range

from 0.61 to Ϫ0.16 ppm. There is, therefore, some influence

both by the solvent and the RO group. In d₆-Me₂SO solu-

Ϫ

tions δ¹H for BH₄is always found at Ϫ0.30 ppm, and the

1:1:1:1 quartet is often accompanied by a septet due to

¹J(¹⁰B¹H) coupling of about 27.3 Hz. The uniformity of the

¹H chemical shifts for the Me₂SO solutions clearly demon-

strate the presence of free BH₄ions. Moreover, the NMR

data exclude any RO^Ϫ/H^Ϫexchange within the

(RO)Mg(BH₄) molecule but allows no conclusion as to the

bonding mode of the BH₄group.

While all (RO)Mg(BH₄) compounds showed only a single

¹¹B NMR resonance, [(2-iPrO)-C₆H₄O]Mg(BH₄)(thf)₂, 24,

in C₆D₆solution exhibited two signals at Ϫ38.5 and

Ϫ42.6 ppm. There are, obviously, two species present. The

simplest explanation would be the assumption of an equili-

brium between a dinuclear and a mononuclear species wi-

thout rapid scrambling between the two. There could also

be two isomers (F and G, see scheme 2) in solution as

shown for mononuclear 24 in scheme 2. On the other hand,

[(2-iPrO)C₆H₄O]Mg(BH₄)(OEt₂), 13, crystallized from

Me₂SO as [(2-iPrO)C₆H₄O]₂Mg-(OSMe₂)]₂, 45, showing

that a ligand exchange is possible (Equation (10)). In any

case no rapid exchange between the two BH₄^Ϫspecies is in-

dicated.

No IR bands higher than 2400 cm^Ϫ1have been observed

for the dme and diglyme solvates. This shows that tridentate

BH₄groups are not present. Thus, the models of type I and

K may represent [20] the structures of the dme and diglyme

1 : 1 solvates of (RO)Mg(BH₄), respectively. The numerical

values of the IR spectra are reported in [20].

Solvates of Bis(trimethylsilylamino)magnesium

Tetrahydroborates

Aminomagnesium tetrahydroborates have almost not been

investigated, and only bis(2,6-diisopropyl-β-diketoiminate

magnesium tetrahydroborate carrying a tridentate BH₄

group is known [21]. Although primary and secondary ami-

nes may react in different ways with EtMg(BH₄) as shown

in equations (11) to (13) we observed only the reaction ac-

cording to (11) on treatment of EtMg(BH₄) with (Me_3-

Si)₂NH.

IR Spectra

EtMgBH₄ϩ (Me₃Si)₂NH Ǟ (Me₃Si)₂NMgBH₄ϩ EtH

EtMgBH₄ϩ (Me₃Si)₂NH Ǟ EtMg[(BH₃N(SiMe₃)₂] ϩ H₂

EtMgBH₄ϩ (Me₃Si)₂NH Ǟ Me₃SiNHMgBH₄ϩ Me₃SiH

(11)

(12)

(13)

As the ¹¹B NMR spectra are inconclusive regarding the

bonding mode of the BH₄group to the Mg centre, we ho-

ped that IR. spectra might provide the data necessary to

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689

M. Bremer, G. Linti, H. Nöth, M. Thomann-Albach, G. E. W. J. Wagner

1

Table 3 ¹¹B and H chemical shifts (ppm), J(¹¹B¹H)coupling con-

stants (Hz) in deuterobenzene of the bis(trimethylsilylamino)ma-

gnesium tetrahydroborates

RX

L

δ¹¹

B

¹J(¹¹B¹H)

δ¹H

¹J(¹¹B¹H)

(Me₃Si)₂N

Me₃SiO

(Me₃Si)₂N

OEt₂, 46

OEt₂, 50

thf, 47

dme, 48

diglyme, 49

Ϫ37.8

Ϫ43.8

Ϫ39.9

Ϫ39.1

Ϫ41.9

br

Ϫ0.37

Ϫ0.39

Ϫ0.41

82.3

82.4

82.2

81.4

82.5

81.4

81.8

81.4

Scheme 4

We favor the first of these three structures because only

one type of (Me₃Si)₂N and dme groups are observed in its

¹H NMR spectrum.

Hexamethyldisilazane has been chosen as an appropriate

secondary amine because the (Me₃Si)₂N group generally

provides good solubility of its derivatives. [(Me₃Si)₂N]-

Mg(BH₄), results also by reacting NaBH₄with [(Me₃Si)₂N]-

MgCl (only checked by NMR). [(Me₃Si)₂N]Mg(BH₄)-

(OEt₂), 46, crystallizes from diethyl ether. The coordinated

diethyl ether can be replaced by thf, dme, and diglyme, gi-

ving compounds 47 to 49.

Hexacoordination at Mg is achieved for 49 by a triden-

tate diglyme ligand. Wheras only one set of signals for the

MeOCH₂CH₂O units of the diglyme molecule is observed

1

in the H and ¹³C NMR spectra there were two signals for

the (Me₃Si)₂N group. This indicates hindered rotation ab-

out the Mg-N bond.

Compounds 46 to 50 show 1:4:6:4:1 quintets in their ¹¹B

NMR spectra, and 1:1:1:1 quartets in the ¹H NMR spectra,

typical for the tetrahydroborate group. The ¹¹B chemical

[(Me₃Si)₂N]Mg(BH₄)(OEt₂)₂

[(Me₃Si)₂N]Mg(BH₄)(thf)₂

shifts depend on the ether ligand while the J(¹¹B¹H) cou-

1

46

47

pling constant is independent. The increase in the shielding

of the ¹¹B nucleus is in accord with an increase of charge

separation along the series 46 to 49 as suggested by Gut-

mann [23], because the increasing donor strength of the sol-

vent molecule will weaken the MgH₂B interaction.

[(Me₃Si)₂N]Mg(BH₄)(dme)

48

[(Me₃Si)₂N]Mg(BH₄)(diglyme)

[(Me₃SiO)Mg(BH₄)(OEt₂)₂],

49

50

Molecular Structure of 49

Although all these compounds were crystalline, they dif-

fracted only very weakly. So their structures could not be

reliably determined. Repeated crystallisation of 46 from

diethyl ether gave crystals that proved to be dimeric 50 (see

Fig. 5).

As can be seen in Figure 9 the BH₄group binds as a biden-

tate ligand to the Mg atom. The bridging hydrogen atoms

form longer bonds to the B atom than the terminal ones.

Hexacoordination at Mg is achieved by the diglyme mole-

cule which occupies, in a distorted fashion, the meridional

sites of the complex. As expected the HϪMgϪH bond an-

gle is quite sharp (71.3°). This is also true for the

O1ϪMg1ϪO2 bond angle which is 72.4(3)°, wheras the

O1ϪMg1ϪO1A bond angle is quite wide (140.0(3)°). Thus

the octahedral geometry around the hexacoordinated Mg

atom is distorted.

Spectroscopy

1

Table 3 lists relevant H and ¹¹B chemicals shifts as well

1

as J(¹¹B¹H) coupling constants. ²⁵Mg chemical shifts have

already been reported [22]. The strong BH band at

2438 cm^Ϫ1and two strong bands at 2288 and 2244 cm^Ϫ1

for 46 indicate the presence of a tridentate BH₄group. This

would be in accord with a dimeric molecule having a hexa-

coordinated Mg atom.

Although all compounds 46 to 50 are crystalline, their

diffracting power as mentioned above was quite weak and

no reflections were observed at 2θ angles higher than 30°

except for compound 49 and 50.

Thus, it is most likely that compound 46 is dinuclear in

the crystal, too, in analogy to [(Me₃SiO)Mg(BH₄)(OEt₂)]₂,

Not unexpected is the planar geometry around its N

50. Since

4

BH stretching bands in the region

atom. Its MgϪO bond lengths are different. Thus, the cen-

2350Ϫ2180 cm^Ϫ1have been observed for 48 and 49, these

compounds should have bidentate BH₄units in order to

achieve hexacoordination at the Mg centers. This has been

ascertained by the X-ray structure of mononuclear 49 (Mg

coordinated to three O, one N and two H atoms). There-

fore, one can expect that 48 is dimeric with bridging (Me_3-

Si)₂N groups between the Mg atoms. Possible structures are

shown in scheme 4.

tral MgϪO2 bond is much shorter (2.141 A) than the bonds

˚

to the MeO groups (2.175(5) A). Although the standard de-

viation of the MgϪO2 bond is twice that of the Mg1ϪO1

bond, no disorder could be detected. The molecular data

correspond nicely with those found for Mg[N(SiMe₃)₂](thf)₂

˚

(SiϪN 1.702, 1-709 A, SiϪNϪSi 120.2, 121.7°) [24] but de-

viate strongly compared with those of K[N(SiMe₃)₂](dio-

˚

xane)₂(NϪSi 1.64 A, NϪSiϪN 136.2°) [25].

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Metal Tetrahydroborates and Tetrahydroborato Metalates

with the determined structures of compound 35. It is the

only example of a compound with two different ether mole-

cules as ligands. Its (2,6-di(tert-butyl)phenolato group is al-

most linearly bonded to the Mg centre, and the BH₄group

is bidentate. Compounds of type (RO)Mg(BH₄)(dme) may

contain a dme molecule that bridges two Mg atoms. Thus

each Mg atom would coordinate with three O atoms, two

from two dme molecules and one from the bridging OR

group. Consequently, the BH₄group should be of a triden-

tate type. However, the IR data do not comply with this

assumption. In case of a bidentate BH₄group, the Mg atom

would be pentacoordinated.

For the diglyme solvates of (RO)Mg(BH₄(diglyme)_nthe

1:1 ratio dominates. Provided that all O atoms of diglyme

coordinate to the Mg atom, then all compounds of this type

should be monomeric with bidentate BH₄groups. This fits

with the IR data. Moreover, this assumption is also suppor-

ted by the structure of the mononuclear compound [(Me_3-

Si)₂N]Mg(BH₄)(diglyme), 49.

Figure 8 Molecular structure of [(Me₃Si)₂N]Mg(BH₄)(diglyme),

(49).

Selected bond lengths/A: MgϪN 2.013(7), NϪSi1 1.698(5), NϪSi2

1.711(7), MgϪO1 2.175(5), MgϪO2 2.14(1), MgϪO1A 2.175(5),

BϪH10 1.07(5), MgϪH10 2.09(4), BϪH11 1.06(6), BϪH12

˚

0.93(6).

Ϫ

Selected bond angles/°: O1ϪMgϪO2 72.4(2),

O1ϪMgϪO1A 140.0(3), Si1ϪNϪSi2 122.6(4), MgϪNϪSi1

117.0(4), MgϪNϪSi2 120.4(3), NϪMgϪB 120(4), H10ϪBϪH12

103(3), H11ϪBϪH12 120(5), H10ϪBϪH11 109(3).

In general, all compounds reported here show no ten-

dency in their respective ether solution to exchange their

substituents according to

Discussion

2 (RO)Mg(BH₄) v (RO)₂Mg ϩ Mg(BH₄)₂

(13)

The structure of solvent-free Mg(BH₄)₂is still unknown.

However, several solvates of this compound have been pre-

pared and structurally characterized. Apart from

Mg(BH₂)₄(thf)₃[6Ϫ8] with a heptacoordinated Mg atom,

almost all of its coordination compounds with amines are

mononuclear with a hexacoordinated Mg atom [1]. An ex-

ception is Mg(BH₄)₂)(py)₄with an octacoordinated Mg

centre. Another compound, Mg(BH₄)₂(OSMe₂)₆, is also ex-

ceptional in so far as it has an ionic structure [Mg(OS-

Me₂)₆](BH₄)₂[1]. In the present study a number of ether

solvates of alcoholato- and phenolato magnesium tetrahy-

droborates have been prepared, and characterized by NMR

and IR spectroscopy, and eight of them by X-ray diffraction

methods. All compounds of type (RO)Mg(BH₄)(OEt₂) are

dimeric with a centrosymmetric Mg₂O₂core, making the

Mg atoms hexacoordinated. Those of type (RO)Mg(B-

H₄)(OEt₂)₂(RO ϭ 2-isopropoxyphenolato, 13, or 2,6-di-

tert-buytlphenolato, 12) are monomeric. In case of 12, the

phenolato group is almost linearly bound to the Mg centre,

and its BH₄group is tridentate. Again hexacoordination for

the Mg atom is achieved. Although the structure of 14 has

not been determined by X-ray crystallography, it is most

likely, that the complex is dimeric with Mg atoms coordina-

ted to four O and two H atoms.

This has been observed only for [(2-iPrO)C₆H₄O]-

Mg(BH₄)(thf) 24, in Me₂SO solution (see Eq.(9)). And this

may be due to the fact that Me₂SO replaces the BH₄group,

thus inducing the ligand exchange. But there remains the

question why this is observed only for this compound. The

resulting {[2-iPrO)C₆H₄O]]₂Mg(OSMe₂)}₂is one of the few

characterized magnesium phenolates. Other examples are

[Mg(OC₆H₃(tBu₂)]₂[26], (Mg(O-Tol)(thf)₂[27], [(2,6-

iPr₂C₆H₃O)₂Mg]₃[28] or Mg(OC₆H₃-(2,6-Me₂)(py)₃[28].

Coordination numbers 3, 4, and 5 are realized in these com-

pounds.

In contrast to many of the (RO)Mg(BH₄) compounds the

silyl amide [(Me₃Si)₂N]Mg(BH₄) is much better soluble in

ethers. In this case, like the RO groups, the silylamido group

occupies the bridging position in the dimeric molecules 46

and the BH₄is bidentate. In case of mononuclear 49 the

tridentate diglyme prevents the formation of a bridging

amino group. In no case was it possible to observe the BH₄

group in a bridging position between Mg atoms. This, ho-

wever, can be expected for compounds of type

Mg(BH₄)₂(L) (L ϭ Et₂O, thf), and of course also for

MgH(BH₄)(L), and particularly for solvent-free Mg(BH₄)₂.

The base thf adds to (RO)Mg(BH₄) compounds in ratios

1 : 1 (18, 19, 20, 25), 1 : 1.5 (17), 1 : 2 (16, 21, 23, 24) or

even 1 : 3 (22). For the 1:1 complexes, one can assume that

the Mg atoms are coordinated by three O atoms and a tri-

dentate BH₄group. As shown for (PhO)Mg(BH₄)(thf)₂, 21,

this compound is centrosymmetric with hexacoordinated

Mg atoms and bidentate BH₄groups. On the other hand,

it seems logical that the compound (2,6-iPr₂C₆H₃O)-

Mg(BH₄)(thf)₃, 22, is monomeric, having a bidentate BH₄

group to complete hexaco-ordination. This would be in line

Experimental

All experiments were conducted under anhydrous conditions and

under a blanket of dinitrogen gas using Schlenk techniques. Ether

solvents were dried by conventional methods, distilled and stored

under N₂. Usually, LiAlH₄was employed as the drying agent.

NMR: Bruker WP 200 and/or 400 using tms, BF₃·OEt₂as stan-

dards for ¹H, ¹¹B and ¹³C NMR spectra. Coupling constants are

given in Hz, chemical shifts in ppm. IR: Perkin Elmer 325 instru-

ment, nujol mulls, KBr plates. X-ray structure determinations: Sie-

Z. Anorg. Allg. Chem. 2005, 631, 683Ϫ697

zaac.wiley-vch.de

691

M. Bremer, G. Linti, H. Nöth, M. Thomann-Albach, G. E. W. J. Wagner

mens P4 diffractometer operated with MoKα radiation, graphite

monochromator. LT2 low temperature device, scintillation detector,

ω/2θ-scans. Abbreviation used: s ϭ singlet, d ϭ doublet, t ϭ triplet,

quart ϭ quartet, quint ϭ quintet, sept ϭ septet.

δ¹H ϭ 0.94 (t, ³J(¹H¹H) ϭ 7.0, CH₂CH₃), 1.23 (d, ³J(¹H¹H) ϭ

7.0), 4.10 (sept, ³J(¹H¹H) ϭ 6.2, CH). Ϫ C₇H₂₁O₂BMg (172.36):

calc. Mg 14.10, B 6.72, H^Ϫ2.34; found Mg 14.52, B 6.34, H^Ϫ

2.30 %.

Preparation of a stock solution of EtMg(BH₄) in diethyl ether: A

Grignard solution of EtMgBr in diethyl ether was prepared by

standard procedures, and 500 mL of the solution containing

500 mmol of the Grignard reagent was transferred into a Schlenk

vessel containing a stirring bar. 40 g of finely powdered NaBH₄

were added and the suspension stirred for 3 h at ambient tempera-

ture until a test showed that the solution was free of bromide. Then

the insoluble material was removed by centrifugation. The solid

material was treated twice with 100 mL of diethyl ether. About

250 mL of the solvent were removed in vacuo and the concentra-

tion of EtMg(BH₄) solution determined after hydrolysis by analysis

of Mg^2ϩ, B(OH)₃and H^Ϫ. The stock solutions used showed con-

centrations between 1 and 1.5 mol/L.

Tert-butylato-magnesium tetrahydroborate diethyl ether, (4): Prepa-

red from a diethyl ether solution of EtMgBH₄(17.3 mL, 1.54 M,

26.6 mmol) and tert-butyl alcohol (2.50 mL, 26.6 mmol), dissolved

in diethyl ether (150 mL). Yield: 1.55 g (31.3 %). NMR (in C₆D₆):

3

δ¹H ϭ 0.92 (t, J(¹H¹H) ϭ 7.0, H₃C-CH₂, 4 H), 1.37 (s, 9H), 3.52

(quart, ³J(¹H¹H)ϭ 7.0, 4 H). Ϫ δ¹³C ϭ 14.0, 65.2 (Et₂O), 34.2

(H₃CC), 68.9 ppm (H₃CC). Ϫ C₈H₂₃BO₂Mg (186.39): calc. Mg

13.04, B 5.80, H^Ϫ2.16; found Mg 14.30, B 5.74, H^Ϫ2.35 %.

Triethylmethylato-magnesium tetrahydroborate diethyl ether, (5):

Prepared from EtMg(BH₄) (77.0 mL, 1.30 M, 100 mmol) and Et_3-

COH (11.62 g, 100 mmol) dissolved in diethyl ether (200 mL).

Yield: 17.3 g (75.3 %), m. p. >400 °C (dec.). NMR (in C₆D₆):

δ¹H ϭ 0.81 (t, ³J(¹H¹H) ϭ 7.6, 6H, CH₂), 1.11 (t, ³J(¹H¹H) ϭ

Elemental analysis: Mg^2ϩby titration with EDTA. B(OH)₃after

solvolysis with MeOH and distillation of B(OMe)₃followed by hy-

drolysis and addition of mannitol and acidimetric titration. H^Ϫby

collecting the hydrogen gas evolved after acid hydrolysis of

EtMgBH₄in a high vacuum apparatus using a Toepler pump. The

ethane generated was absorbed on active carbon at Ϫ196 C.

3

7.6 Hz, 9H, CH₃), 1.30 and 1.59 (quart, J(¹H¹H) ϭ 7.5, 6H), 3.27

(quart, ³J(¹H¹H) ϭ 7.6, 4H, CH₂of Et₂O). Ϫ δ¹³C ϭ 15.5, 65.8

(Et₂O), 29.9, 30.0, 30.8 (OC(CH₂), 7.9 (CH₃CH₂)₃C).

Ϫ

C₁₇H₂₉O₂BMg (229.45): calc. Mg 10.64, B 4.73, H^Ϫ1.75; found

Mg 11.21, B 4.77, H^Ϫ1.72 %.

Neopentylato-magnesium tetrahydroborate diethyl ether, (6): Prepa-

red from EtMg(BH₄) (20 mL, 1.05 M, 73.4 mmol) and neopentyl

alcohol (6.47 g, 73.4 mmol) dissolved in diethyl ether (40 mL).

Yield: 2.34 g (16.5 %). The mother liquor has not been worked up

to increase the yield. m. p. 108 Ϫ 109 °C. NMR (in C₆D₆): δ¹H ϭ

0.95 (br, 6 H, Et₂O), 1.01 (br, 9H, Me₃CH₂), 4.40 (br, 6H,

(MeCH₂)₃), Et₂O and Me₃CCH₂O), Ϫ δ¹³C ϭ 4.0 (br, 65.6

Diethyl Ether Solvates of (RO)Mg(BH₄)

To a stirred diethyl ether solution of EtMg(BH₄) was added an

diethyl ether solution of the alcohol or phenol. After gas evolution

has subsided, the solid that has been formed was isolated by filtra-

tion, washed with some diethyl ether and dried shortly in vacuo. If

a clear solution resulted, then diethyl ether was partially stripped

off, the solution cooled to Ϫ20 °C and the solid which formed

was isolated.

(CH₃CH₂O), 26.9 (H₃C), 33.6 (Me₃CH₂), 74.7 (CH₂O).

Ϫ

C₉H₂₅O₂BMg (200.40): calc. Mg 12.13, B 5.39, H^Ϫ2.01; found Mg

12.48, B 5.16, H^Ϫ1.95 %.

Triphenylmethylato؊magnesium tetrahydroborate diethyl ether, (7):

Prepared from EtMg(BH₄) (38.4 mL, 1.30 M, 50 mmol) and

Ph₃COH (13.0 g, 50 mmol) dissolved in diethyl ether (150 mL).

Yield: 13.2 g (70.6 %), m. p. 120 Ϫ 125 °C. NMR (in C₆D₆): δ¹H ϭ

0.96 (t, 6H, Et₂O), 3.26 (quart, 4H, Et₂O), 7.00 Ϫ 7.13, 7.29 Ϫ

7.36, 7.49 Ϫ 7.59 (mult, 15 H). Ϫ δ¹³C ϭ 15.5, 65.9 (OEt₂), 82.1,

87.2 (CPh₃), 126.5, 127.83, 127.87, 128.9, 129.9, 146.6, 147.6 (arom

C). Ϫ C₂₀H₂₉O₂BMg (372.60): calc. Mg 6.52, B 2.90, H^Ϫ1.08;

found Mg 6.68, B 2.87, H^Ϫ0.97 %.

Methylato-magnesium tetrahydroborate diethyl ether, (1): To a stir-

red diethyl ether solution of EtMg(BH₄) (105 ml, 1.08 M,

113 mmol) was dropwise added methanol (4.64 mL, 113 mmol).

Ethane was rapidly generated . A precipitate formed that was isola-

ted by filtration and dried in vacuo. Yield: 8.83 g (54.1 %), m. p.

dec. >400 °C. The compound is slightly soluble in diethyl ether,

benzene, and toluene, but freely soluble in thf (with ether displace-

ment). NMR (in C₆D₆): δ¹H ϭ 3.27 (quart, ³J(¹H¹H) ϭ 7.0,

3

OCH₂), 1.07 (t, J(¹H,¹H) ϭ 7.0, CH₃); 1.35 (s, OCH₃); ratio ϭ 4

2-Methoxyethylato؊magnesium tetrahydroborate diethyl ether, (8):

To a stirred solution of EtMg(BH₄) in diethyl ether (105 M,

19.3 mL) was added MeOCH₂CH₂OH (1.54 g, 1.60 mL, 20.3 mol).

After gas evolution ceased the precipitate was isolated. Yield: 1.08 g

(46.6 %), m. p. 195 Ϫ 198 °C. NMR (in d₆-Me₂SO ): δ¹H ϭ 3.80

Ϫ 4.60 ppm (m, 7 H, CH₂, CH₃). Ϫ δ¹³C ϭ 58.1, 58.7, 60.6, 61.3

(MeO(CH₂)₂), 15.0, 65.1 (Et₂O). Ϫ C₇H₂₁O₃BMg (114.24): calc.

Mg 21.28, B 9.46, H^Ϫ3.53; found Mg 20.41, B 8.86, H^Ϫ3.08 %.

: 6 : 3. Ϫ δ¹³C ϭ 15.6 (CH₂CH₃), 30.4 (OCH₃), 65.9 (OCH₂), and

weak additional signals at 20.3, 22.7, 27.4, 37.3. Ϫ C₅H₁₇O₂BMg

(144.30): calc. Mg 16.84, B 7.49, H^Ϫ2.80; found Mg 16.64, B 7.43,

H^Ϫ2.80 %.

Ethylato-magnesium tetrahydroborate diethyl ether, (2) : Prepared

in analogy to 1 from a diethyl ether solution of EtMg(BH₄) (93 mL,

1.08 M, 100 mmol) and a solution of ethanol (5.80 mL, 100 mmol)

in diethyl ether (100 mL). Yield: 12.5 g (78 %), m. p. 245 Ϫ 250 °C.

The compound is slightly soluble in diethyl ether, benzene and to-

luene but excellently in thf and Me₂SO. NMR (in d₆-Me₂SO ):

δ¹Hϭ 1.03 (br, CH₃CH₂O^Ϫ, 9 H)), 3.40 (m, br, 2 H), 3.60 (m, br,

4 H). Ϫ δ¹³C ϭ 15.2, 64.9 (OEt₂), 18.7 (H₃CCH₂O^Ϫ), 56.1 (br,

H₃CCH₂O^Ϫ). Ϫ C₆H₁₉O₂BMg (158.33): calc. Mg 15.35, B 6.83,

H^Ϫ2.55; found Mg 15.50, B 6.83, H^Ϫ2.36 %.

Phenolato؊magnesium tetrahydroborate diethyl ether, (9):

EtMg(BH₄) (13.1 mL, 1.54 M, 21.7 mmol) was allowed to react

with phenol (2.05 g, 21.7 mmol) dissolved in diethyl ether

(100 mL). Yield: 2.40 g (53.6 %), m. p. 185 Ϫ 187 °C. NMR (in

C₆D₆): δ¹H ϭ 0.68 (t, ³J(¹H¹H) ϭ 7.1, 6 H, Et₂O), 3.23 (t,

³J(¹H¹H) ϭ 7.1, 4H, Et₂O), 6.72 Ϫ 7.42 (m, 5H). Ϫ δ¹³C ϭ 13.9,

66.3 (OEt₂), 119.5, 130.3, 159.4 ppm (ipso C not detected). Ϫ

C₁₀H₁₉O₂Mg (206.38): calc. Mg 11.78, B 5.24, H^Ϫ1.95; found Mg

11.79, B 4.84, H^Ϫ1.76 %.

Isopropylato-magnesium tetrahydroborate diethyl ether, (3): Prepa-

red in analogy to 1 from EtMgBH₄(13.0 mL, 1.54 M, 20.0 mmol)

and a solution of isopropanol (1.53 mL, 20.0 mmol) in diethyl ether

(150 mL). Yield 1.94 g (56.3 %), m. p. 72 Ϫ 74 °C. NMR (in C₆D₆):

2,4,6-Trimethylphenolato؊magnesium tetrahydroborate diethyl

ether (1/1.5), (10): Prepared in analogy to 1 from EtMg(BH₄)

692

zaac.wiley-vch.de

Z. Anorg. Allg. Chem. 2005, 631, 683Ϫ697

Metal Tetrahydroborates and Tetrahydroborato Metalates

(78.7 mL, 1.27 M, 100 mmol) and 2,4,6-trimethylphenol (13.6 g,

100 mmol) dissolved in diethyl ether (100 mL). A clear solution

resulted. All volatile material was removed in vacuo and the residue

crystallized from a small amount of diethyl ether. Yield: 9.0 g

(36.2 %), m.p. 151 Ϫ 153 °C. NMR (in d₆-Me₂SO ): δ¹H ϭ 1.09

but was only slightly soluble in benzene and almost insoluble in

hexane. NMR (in d₆-Me₂SO): δ¹H ϭ 3.63 (s, 3 H, OCH₃), 3.72 (s,

3

3 H, OCH₃), 6.09 (m, 1 H), 6.42 (d, J(¹H¹H) ϭ 8.3 Hz, 1 H), 6.49

(d, ³J(¹H¹H) ϭ 8.3, 1 H). Ϫ δ¹³C ϭ 55.9, 56.0 (OCH₃), 106.6,

107.2, 128.1, 128.7, 149.2 (arom. C). Ϫ C₈H₁₃O₃BMg (192.31):

calc. Mg 12.64, B 5.63, H^Ϫ2.10 Ϫ found Mg 12.15, B 4.67, H^Ϫ

2.22 %.

3

(t, J(¹H¹H) ϭ 7.1, H₃C-CH₂, 6 H), 3.38 (quart, J(¹H¹H) ϭ 7.1,

MeCCH₂), 4H), 2.10 (s, 6 H, 2,6-Me), 2.12 (s, 3 H, 4-Me), 6.67 (s,

2H), 7.86 (s, 1 H). Ϫ δ¹³C ϭ 15.1, 64.8 (Et₂O), 20.0, 16.5, Me),

123.9, 127.0, 128.5 (C_qrom). Ϫ C₃₀H₅₀O₅B₂Mg₂(571.02): calc. Mg

8.51, B 3.79, H^Ϫ1.41; found Mg 9.73, B 3.65, H^Ϫ1.37 %.

(2,2-Diethyl-3,3-dimethylpropylato-magnesium

tetrahydroborate

diethyl ether, (15): solution of diethylketone (1.84 mL,

A

20.0 mmol) in diethyl ether (20 mL) was dropped into a stirred so-

lution of tBuMg(BH₄) 1.33 OEt₂[29] (3.35 g, 20.0 mmol, 100 mL

Et₂O). Stirring was continued for 1 h. Then all volatile material

was removed in an oil pump vacuo. Yield: 4.67 g, (92.0 %), m. p.

>350 °C. NMR (in C₆D₆): δ¹H ϭ 1.37 (s, (CH₃)₃C), 1.45 (m, CH₃

and CH₂groups), 3.59 (m, broad, CH₂of Et₂O). Ϫ δ¹³C ϭ 14.3,

64.6 (OEt₂), 10.2 (s, H₃C-CH₂), 29.4 (Me-CH₂), 34.2 (H₃C)₃C),

68.6 (Me₃C), 74.8 (C-O). Ϫ C₁₃H₃₃O₂BMg (256.52): calc. Mg 9.47,

B 4.21, H^Ϫ1.57; found Mg 9.14, B 3.89, H^Ϫ1.63 %.

2,6-Diisopropylphenolato؊magnesium tetrahydroborate diethyl

ether, (11): Prepared from a diethyl ether solution of EtMg(BH₄)

(70.0 mL, 1.05 M, 73.4 mmol) and (2,6-iPr₂C₆H₃OH (13.6 g,

73.4 mmol) dissolved in diethyl ether (50 ml). Yield: 13.3 g

(65.2 %), m. p. 140 °C, dec. NMR (in C₆D₆): δ¹H ϭ 0.99 and 1.07

3

(d, J(¹H¹H), 6.8 and 6.6 Hz, each 6 H), 1.09 (t, J(¹H¹H) ϭ 7.1,

6 H, CH₃CH₂), 3.37 (quart, ³J(¹H¹H) ϭ 7.1, CH₂O, 4 H), 3.60

(sept, ³J(¹H¹H) ϭ 6.5, 2 H), 6.15 (t, ³J(¹H¹H) ϭ 7.2, 1 H), 6.65 (d,

³J(¹H¹H) ϭ 7.3, 2 H). Ϫ δ¹³C ϭ 15.3, 65.1 (Et₂O), 23.6 and 23.8

(H₃C)₂CH, ), 25.6, 25.8 (CH₃)₂CH), 110.6, 121.3, 135.6, 135.8,

161.4 (arom C). Ϫ C₁₆H₃O₂BMg (290.52): calc. Mg 8.37, B 3.72,

H^Ϫ1.39; found Mg 8.39, B 3.32, H^Ϫ1.28 .

Tetrahydrofuran Solvates of ROMgBH₄Compounds

General procedure: Diethyl ether solvates of ROMg(BH₄) were dis-

solved in thf. Usually a clear solution formed. Depending on con-

centration crystals of ROMg(BH₄)(thf)_nseparated. These were iso-

lated by filtration. Otherwise the volatile material was removed

from the solutions after some hours, and the solid white residue

analyzed.

2,6-Di(tert-butyl)phenolato؊magnesium tetrahydroborate diethyl

ether, (12): Prepared from EtMg(BH₄) (54.8 mL, 0.73 M,

40.0 mmol) in diethyl ether and (2,6-tBu₂)C₆H₃OH (8.25 g,

40.0 mmol) dissolved in diethyl ether (30 mL). Yield 10.6 g

(69.9 %); m. p. 154 Ϫ 156 °C. NMR (in C₆D₆): δ¹H ϭ 1.02 (t,

³J(¹H¹H) ϭ 6.8, 8 H, H₃CCH₂), 1.35 (s, 18 H, Me₃C), 1.64 (s, 6 H),

3.31 (quart, ³J(¹H¹H) ϭ 6.9, 8 H, CH₂O), 6.85 (t, ³J(¹H¹H) ϭ 7.8),

Methylato-magnesium tetrahydroborate tetrahydrofuran (1/2), (16):

To 20 mL of a diethyl ether solution of (MeO)Mg(BH₄)(OEt₂)

(1.47 g, 10.2 mmol) was added thf (20 mL). Crystals separated from

the solution at Ϫ20 °C. Yield: 1.04 g (57.4 %), m. p. 70 Ϫ 72 °C.

NMR (in C₆D₆): δ¹H ϭ 0.94 (s, 3 H, H₃CO), 1.33 and 2.64 (m,

each 4 H, thf). Ϫ δ¹³C ϭ 25.4, 59.9 (THF), 30.1 (H₃C). Ϫ

C₉H₂₀O₃MgB (214.99): calc. Mg 11.34, B 5.04, H^Ϫ1.88; found Mg

12.74, B 5.64, H^Ϫ2.06 %.

3

7.17 (d, J(¹H¹H) ϭ 7.8) (3 arom H). Ϫ δ¹³C ϭ 30.3, 31.4, 34.3,

35.4 (H₃C)₃C, 14.5, 65.5 (OEt₂), 68.9 (CO), 114.5, 125.3, 136.0,

138.0 (arom C). Ϫ C₁₈H₃₅O₂BMg (382.70): calc. Mg 5.35, B 2.82,

H^Ϫ1.05; found Mg 5.98, B 2.87, H^Ϫ1.07 %.

2-Isopropoxyphenolato؊magnesium tetrahydroborate diethyl ether

(1/2), (13): Compound 13 was obtained by reacting a diethyl ether

solution of EtMg(BH₄) (70.0 mL, 1.05 M, 73.4 mmol) with an

ether solution of 2-isopropoxyphenol (11.12 g, 73.4 mmol) dissol-

ved in diethyl ether (30 mL). Yield: 20.8 g (83.5 %), m. p. 280 °C.

NMR (in d₆-Me₂SO): δ¹H ϭ Ϫ1.09 (t, ³J(¹H¹H) ϭ 7.0), 12 H,

CH₃CH₂O), 1.24 (d, ³J(¹H¹H) ϭ 4.4, 6 H, (CH₃)₂CH), 3:36 (quart,

³J(¹H¹H) ϭ 7.1, 8 H, MeCH₂O), 4.59 (br; 1 H, Me₂CH), 6.75 (m,

4 H_arom.). Ϫ δ¹³C ϭ 15.4, 65.2 (Et₂O), 22.0 (H₃C)₂CH, 70.5

Ethylato-magnesium tetrahydroborate tetrahydrofuran (1/1.5), (17):

EtMg(BH₄)(OEt₂) (1.86 g, 11.7 mmol) was dissolved in thf

(100 mL). About 70 % of the solvent was removed in vacuo. From

the remaining solution crystals separated on cooling in a refrigera-

tor. Yield: 1.43 g (63.5 %), m. p. ϳ20 °C. NMR (in C₆D₆): δ¹H ϭ

1.27 (m, broad, ϳ 8 H, thf and MeCH₂O), 3.64 (m, 5.3 H, thf),

3.86Ϫ3.71 (m, 2H). Ϫ δ¹³C ϭ 25.3, 69.4 (thf), 21.6 (H₃CCH₂), 58.4

(MeCH₂). Ϫ C₁₄H₃₃O₅B₂Mg₂(384.72): calc. Mg 12.64, B 5.62, H^Ϫ

2.10; found Mg 11.56, B 5.63, H^Ϫ2.12 %.

(Me₂CH), 121.7, 127.8, 127.9, 128.1, 128.2 (arom C).

Ϫ

C₁₇H₃₅O₄BMg (318.7): calc. Mg 7.18, B 2.19, H^Ϫ1.19; found Mg

7.38, B 2.26, H^Ϫ1.25 %.

Isopropylato-magnesium tetrahydroborate tetrahydrofuran, (18):

iPrOMg(BH₄)(OEt₂) (2.54 g, 14.7 mmol) was dissolved in 20 mL of

thf. A precipitate formed on storing the solution at Ϫ20 °C. Yield:

1.24 g, (49.5 %), m. p. 79 Ϫ 80 °C. NMR (in C₆D₆): δ¹H ϭ 1.22

(d, ³J(¹H¹H) ϭ 6.1, 6 H, (H₃C)₂C), 1.34 (m, 4 H), thf), 3.68 (m,

2,6-Dimethoxyphenolato؊magnesium tetrahydroborate diethyl ether,

(14): Prepared from a diethyl ether solution of EtMg(BH₄)

(89.0 mL, 0.95 M, 84.3 mmol) and

a diethyl ether solution

(110 mL) of 2,6-dimethoxyphenol (13.0 g, 84. 3 mmol). Yield:

20.9 g (93.2 %), m. p. 300 °C, dec.. NMR (in d₆-Me₂SO): δ¹H ϭ

1.09 (m, 6 H, H₃CCH₂), 3.38 (m, 4 H, CH₂O), 3.64 and 3.73 (two

s, each 3 H, OCH₃), 6.09 (dd, 1 H, 6.41 (dd, not resolved, 1 H),

6.41 (d, ³J(¹H¹H) ϭ 8.3), 6.49 (d, ³J(¹H¹H) ϭ 7.8, 1 H). Ϫ δ¹³C ϭ

15.2, 64.9 (Et₂O), 55.64, 56.08 (OMe), 106.7, 107.3, 108.1, 148.5,

149.3 (arom C). Ϫ C₁₂H₂₃O₃BMg (266.38): calc. Mg 9.12, B 4.06,

H^Ϫ1.51; found Mg 9.19, B 4.45, H^Ϫ1.67 %.

3

4 H), 4.12 (sept, 1 H, J(¹H¹H) ϭ 6.1). Ϫ δ¹³C ϭ 25.3, 69.4 (thf),

28.4 ((H₃C)₂C), 63.8 (CH). Ϫ C₇H₁₉O₂BMg (170.34): calc. Mg

14.23, B 6.35, H^Ϫ2.37; found Mg 12.91, B 7.02, H^Ϫ2.61 %.

Tert-butylato-magnesium tetrahydroborate tetrahydrofuran, (19):

Crystals separated from a solution of (tBuO)Mg(BH₄)(OEt₂)

(1.67 g, 5.74 mmol) in thf (50 mL) on storing in a refrigerator.

Yield: 1.67 g (74.2 %, m. p. 214 Ϫ 216 °C. NMR (in C₆D₆): δ¹H ϭ

1.15 (m, 4H, THF), 1.35 (s, 8 H, (H₃C)₃C), 3.67 (m, 4 H, thf). Ϫ

δ¹³C ϭ 25.0, 69.6 (thf), 34.1 (H₃C)₃C), 69.6 (C-O). Ϫ C₈H₂₁O₂BMg

(187.37): calc. Mg 13.18, B 5.86, H^Ϫ2.19; found Mg 11.30, B 5.30,

H^Ϫ2.21 %.

2,6-Dimethoxyphenolato؊magnesium tetrahydroborate, (14a): To

compound 14 (2.90 g, 10.9 mmol) was added toluene (100 mL).

From the suspension the toluene was distilled off at normal pres-

sure. This produced solvent free 14 ϭ 14a. Yield 2.10 g (100 %), m.

p. 350 °C (dec.). The compound dissolved readily in thf or Me₂SO

Z. Anorg. Allg. Chem. 2005, 631, 683Ϫ697

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693

M. Bremer, G. Linti, H. Nöth, M. Thomann-Albach, G. E. W. J. Wagner

(Triethyl)methylato؊magnesium tetrahydroborate tetrahydrofuran,

(20): (Et₃CO)Mg(BH₄)(OEt₂) (2.90 g, 12.6 mmol) was dissolved in

thf (150 mL). After 1 h 70 Ϫ 75 % of the solvent was removed in

vacuo. Crystal separated from the remaining solution on standing

in a refrigerator. Yield: 1.91 g (66.6 %), m. p. 145 Ϫ 147 °C. NMR

(in C₆D₆): δ¹H ϭ 0.96 (t, ³J (¹H¹H) ϭ 7.5, 9 H), 1.21 (br, 4 H, thf),

1.35 (s.3H), 1.76 (s, 3 H), 3.76 (br, 4 H, thf). Ϫ δ¹³C ϭ 25.0, 69.5

(thf), 9.2 (H₃CCH₂), 34.1 (H₃C), 75.1 (CO). Ϫ C₁₁H₂₇O₂BMg

(226,44): calc. Mg 10.73, B 4.77, H^Ϫ1.77; found Mg 10.87, B 4.77,

H^Ϫ1.80 %.

Ethylato؊magnesium tetrahydroborate dimethoxyethane, (26):

EtOMg(BH₄)(OEt₂) (2.18 g, 13.8 mmol) was added to dme

(100 mL) with stirring, After 3 h the insoluble material was isolated

by filtration, washed with some dme and dried in vacuo. Yield:

1.8 g (74.9 %), m. p. 185 Ϫ 188 °C. NMR (in C₆D₆): δ¹H ϭ 0.92

(m, br, 2 H, MeCH₂O), 1.34 (br, 3 H, H₃CCH₂), 3.11 (s, 6 H,

H₃CO), 3.22 (s, 4 H, H₂CO). Ϫ C₆H₁₉O₃BMg (174.33): calc. Mg

13.94, B 6.20, H^Ϫ2.31; found Mg 13.00, B 5.62, H^Ϫ2.11 %.

Isopropylato؊magnesium tetrahydroborate dimethoxyethane, (27):

Obtained as a white powder from (iPrO)Mg(BH₄)(OEt₂) (2.49 g,

14.4 mmol) and dme (15 mL). Yield: 1.50 g (55.3 %), m. p. 300 Ϫ

305 °C. NMR (in C₆D₆): δ¹H ϭ 1.08, 1.17, 1.36 (all d, ³J(¹H¹H) ϭ

6.1 (H₃C)₂CH), 3.14, 3.26 ( both s, H₃COCH₂), 4.27, 4.54 (both

m., Me₂CH). Ϫ δ¹³C ϭ 58.9, 71.9 (H₃COCH₂). The iPrO group

was not clearly observed). Ϫ C₉H₂₁O₃BMg (188.51): calc. Mg

12.89, B 5.73, H^Ϫ2.14; found Mg 13.16, B 5.13, H^Ϫ2.27 %.

Phenolato؊magnesium tetrahydroborate tetrahydrofuran (1/2), (21):

9 (0.78 g, 3.8 mmol) was dissolved in thf (75 mL). After stirring

for several hours the insoluble part was isolated by filtration and

analyzed. Yield: 0.63 g (60 %), m. p. 270 °C dec.. NMR (in C₆D₆):

δ¹H ϭ 1.14 (m, 8 H), 3.63 (m, 8 H), 6.14 Ϫ 6.89 (m, 1 H), 7.16 Ϫ

7.64 (m, 4 H). Ϫ δ¹³C ϭ 25.4, 68.7 (thf), 119.1, 120.3, 129.8, 161.2

(arom C). Ϫ C₁₄H₂₅O₃BMg (276.45): calc. Mg 8.79, B 3.91, H^Ϫ

1.46; found Mg 9.44, B 4.09, H^Ϫ1.40 %.

Tert-butylato؊magnesium tetrahydroborate dimethoxyethane (2/1),

(28): To a stirred solution of (tBuO)Mg(BH₄)(OEt₂) (0.85 g,

4.56 mmol) in diethyl ether (50 mL) was added dme (25 mL). A

white precipitate formed immediately. It dissolved in thf but is only

sparingly soluble in benzene. Yield: 0.46 g (64.1 %), m. p. 146 Ϫ

148 °C. NMR (in C₆D₆): δ¹H ϭ 1.04 (s., 9 H, (H₃C)₃C), 3.11 (s.,

3 H, H₃CO), 3.32 (s, 2 H, H₂CO). Ϫ δ¹³C ϭ 30.9 (H₃C), 59.7, 72.1

(dme), signal for Me₃C not observed). Ϫ C₆H₁₈O₂BMg (157.31):

calc. Mg 15.35, B 6.87, H^Ϫ2.56; found Mg 13.24, B 7.54, H^Ϫ

2.80 %.

2,6-(Diisopropyl)phenolato؊magnesium tetrahydroborate tetrahy-

drofuran (1/3)), (22): Prepared form ((2,6-iPr₂)C₆H₃O)-

Mg(BH₄)(OEt₂) (2.91 g, 10.0 mmol) and thf (20 mL). Yield: 3.98 g

(92.0 %), m. p. 58 Ϫ 62 °C. NMR (in C₆D₆): δ¹H ϭ 1.33 (d,

³J(¹H¹H) ϭ 7.3, 12 H, Me₂C), 1.36 (m, 12 H, thf), 3.75 (m, 12 H,

thf), 3.78 (m, 2H Me₂CH), 6.98 (m, 1 H, p-H), 7.16 (m, 2 H, m-

H). Ϫ C₂₄H₄₅O₄BMg (432.73): calc. Mg 5.62, B 2.50, H^Ϫ0.93;

found Mg 6.08, B 2.52, H^Ϫ0.94 %.

(2,6-Di-tert-butyl)phenolato؊magnesium tetrahydroborate tetrahy-

drofuran (1/2), (23): (2,6-tBu₂C₆H₃O)Mg(BH₄)(OEt₂) (3.02 g,

7.89 mmol) was dissolved in thf (50 ml). Crystals separated from

the concentrated solution on standing in a refrigerator. Yield:

2.10 g, (68.7 %), m. p. 114 Ϫ 118 °C. NMR (in C₆D₆): δ¹H ϭ 1.13

Triethylmethylato؊magnesium tetrahydroborate dimethoxyethane

(2/1), (29): A suspension made from (Et₃CO)Mg(BH₄)(OEt₂)

(1.85 g, 8.06 mmol) in dme (10 mL) was heated to reflux. From the

clear solution 29 separated on cooling as a colorless precipitate.

Yield: 0.80 g (45.9 %), m. p. 300 Ϫ 305 °C, dec.. NMR (in C₆D₆):

3

(m, 8 H), 1.40 (s, 18 H, Me₃C), 3.37 (m, 8 H), 6.60 (t, J(¹H¹H) ϭ

3

δ¹H ϭ 0.76, 0.82, 0.90 (three t, all with J(¹H¹H) ϭ 7.5 Hz, 9 H,

7.7, 2 H), 6.94 (s), 7.22 (d, ³J(¹H¹H) ϭ 7.7), intensity of the last

three signals 3 H). Ϫ δ¹³C ϭ 25.2, 69.4 (THF), 30.4, 31.0, 31.9,

35.3 (H₃C-C), 114.0, 120.3, 125.0, 137.8, 162.9 (arom C). Ϫ

C₂₂H₄₁O₃BMg (387.64): calc. Mg 6.27, B 2.79, H^Ϫ1.04; found Mg

5.84, B 2.60, H^Ϫ0.89 %.

H₃CH₂), 1.32, 1.59, 1.63 (three quart, ¹J(¹H¹H) ϭ 7.5, 6 H,

MeCH₂), 3.09 (s, 3 H, H₃CO), 3.25 (s, 2 H, H₂CO). Ϫ δ¹³C ϭ 7.9,

8.9, 10.5 (H₃CCH₂), 30.7, 32.8, 34.5 (H₃CCH₂), 59.6, 72.1

(H₃COCH₂), 75.5 (Et₃C). Ϫ C₉H₂₄O₂BMg (216.45): calc. Mg

12.18, B 5.42, H^Ϫ2.02; found Mg 11.52, B 4.96, H^Ϫ2.24 %.

2-Isopropoxyphenolato؊magnesium tetrahydroborate tetrahydrofu-

ran (1/2), (24): Prepared from 13, (3.96 g, 11.7 mmol) and thf

(70 mL). Yield: 2.62 g (67.0 %), m. p. 230 Ϫ 235 °C dec.. NMR (in

2-Methoxyethylato؊magnesium tetrahydroborate dimethoxyethane

(2/1), (30): Prepared from (MeOCH₂CH₂O)Mg(BH₄)(OEt₂) (2.6 g,

22.8 mmol) and dme (20 mL). The colorless powder is slightly solu-

ble in dme and highly soluble in Me₂SO. Yield: 2.84 g (71.5 %), m.

p. 110 °C. NMR (in d₆-Me₂SO): δ¹H ϭ 0.57 (m, 2 H), 3.15 to 3.80

(several m, 11 H). Ϫ δ¹³C ϭ 16.3 16.7, 17.2, 17.6 (all weak), 58.1,

71.1 (dme). Ϫ C₁₇H₅₃O₁₀B₃Mg₃(522.93): calc. Mg 13.94, B 6.20,

H^Ϫ2.31; found Mg 14.04, B 6.06, H^Ϫ2.26 %.

3

d₆-Me₂SO) : δ¹H ϭ 1.22 (d, J(¹H¹H) ϭ 5.6, 6 H, Me₂CH), 1.75

3

(br, 4 H, thf), 3.59 (br, 4 H, thf), 4.50 (sept, J(¹H¹H) ϭ 5.6, 1 H,

CHMe₂), 6.6 Ϫ 6.8 (arom. H). Ϫ δ¹³C ϭ 25.3, 68.6 (thf), 20.9

(Me₂CH), 72.8 (CO), 111.9, 117.0, 120.0, 122.9, 152.0, 154.0 (arom

C). Ϫ C₁₇H₃₁O₄BMg (334.55): calc. Mg 7.27, B 3.23, H^Ϫ1.21;

found Mg 9.09, B 2.99, H^Ϫ1.15 %.

Phenolato؊magnesium tetrahydroborate dimethoxyethane (2/3),

(31): On heating a suspension of (PhO)Mg(BH₄)(OEt₂) (1.72 g,

8.33 mmol) in dme (10 mL) a clear solution resulted. On storing it

in a refrigerator at Ϫ4 °C a white microcrystalline precipitate for-

med within 3 d. Yield: 0.53 g (23.9 %), m. p. 180 °C. NMR (in

C₆D₆): δ¹H ϭ 2.90, 3.04 (two s, 15 H, H₃COCH₂), 6.85 (t), 7.27

(t), 7.39 (d, together 5 H). Ϫ C₂₄H₄₈O₈B₂Mg₂(535.16): calc. Mg

9.10, B 4.05, H^Ϫ1.51; found Mg 10.77, B 3.96, H^Ϫ1.55 %.

2,6-Dimethoxyphenolato؊magnesium tetrahydroborate tetrahydro-

furan, (25): Obtained from 14 (2.22 g, 8.33 mmol) and thf

(50 mL).Yield: 1.60 g (72.7 %), m. p. 320 °C, dec.. NMR (in d₆-

Me₂SO): δ¹H ϭ 1.76 (m, 4 H, thf) 3.61 (m, 4 H, thf), 3.74 (s, 3 H,

MeO), 3.78 (s, 3 H, MeO), 6.09 (m, 1 H), 6.41 (d, ³J(¹H¹H), 7.8,

1 H), 6.49 (d, ³J(¹H¹H) ϭ 7.8, 1 H). Ϫ δ¹³C ϭ 25.0, 66.9 (thf), 55.6

and 56.0 (MeO), 106.7, 107.2, 108.0, 148.5, 149.4 (arom. C). Ϫ

C₁₂H₂₁O₄BMg (264.38): calc. Mg 9.19, B 4.09, H^Ϫ1.52; found Mg

9.29, B 3.97, H^Ϫ1.31 %.

(2,6-Diisopropyl)phenolato؊magnesium tetrahydroborate dimetho-

xyethane, (32): To a 1 M ether solution of (iPr₂C₆H₃O)Mg-

(BH₄)(OEt₂) (15 mL) was added with stirring dme (10 mL). The

resulting precipitate was isolated. Yield: 2.46 g (53.6 %), m. p. 206

Dimethoxyethane Solvates of ROMgBH₄

3

These compounds were prepared from ROMgBH₄·OEt₂by ether

displacement. Most of the compounds are only slightly soluble in

dme and benzene.

Ϫ 209 °C. NMR (in d₆-Me₂SO): δ¹H ϭ 0.99 (d, J(¹H¹H) ϭ 6.3,

4 H) and 1.13 (d, ³J(¹H¹H) ϭ 5.4, 8 H, (H₃C)₂CH), 3.24 (s, 6 H,

H₃CO), 3.42 (s, 4 H, H₂CO), 3.60 (m, 2 H, Me₂CH), 6.66 (d,

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Z. Anorg. Allg. Chem. 2005, 631, 683Ϫ697

Metal Tetrahydroborates and Tetrahydroborato Metalates

³J(¹H¹H) ϭ 6.8, 1 H, p-H), 6.92 (br, 2 H, m-H). Ϫ δ¹³C ϭ 16.3,

1.70, 17.1, 17.5 (CH₃)₂C), 23.3, 23.6, 25.4, 25.9 (Me₂C), 58.0, 71.1

(dme), 110.4, 121.3, 135.3, 161.2 (arom. C). Ϫ C₁₆H₃₁O₃BMg

(306.53): calc. Mg 7.93, B 3.53, H^Ϫ1.23; found Mg 8.33, B 3.40,

H^Ϫ1.20 %.

Tert-butylato؊magnesium tetrahydroborate diglyme (3/1), (38):

tBuO)Mg(BH₄)(OEt₂) (2.77 g, 14.0 mmol) was suspended in di-

glyme (20 mL) and the mixture kept at reflux for 1 h. After cooling

the solid was isolated by filtration. Yield: 1.70 g (63.8 %), m. p.

>350 °C, dec.. NMR (in C₆D₆): δ¹H ϭ 1.04 (s, 25 H, (H₃C)₃CO),

3.11 (s, 6 H), 3.35 (m, 4 H), 3.45 (m, 4 H) diglyme. Ϫ δ¹³C ϭ 58.6,

70.9, 72.3 (diglyme), tBuO not detected. C₁₈H₅₂O₆B₃Mg₃(469.35):

calc. Mg 15.51, B 6.01, H^Ϫ2.57; found Mg 17.30, B 5.91, H^Ϫ

2.20 %.

2,5-Di(tert-butyl)phenolato؊magnesium tetrahydroborate tetrahy-

drofuran dimethoxy-ethane, (33): 2,6-tBuC₆H₃OMgBH₄·2OEt₂

(2.75 g, 7.20 mmol) was dissolved in thf (14 mL). Then dme

(3.7 mL) was added with stirring. No precipitate formed, also not

on cooling. Therefore, about 70 % of the solvents were distilled off

in vacuo. Yellow crystals separated on standing for some days at

Ϫ20 °C. Yield: 1.07 g (38.0 %), m. p. 114 Ϫ 120 °C. NMR (in

C₆D₆): δ¹H ϭ 1.35 (s, br, 9 H, (H₃C)₃C), 1.40 (m, 4 H, thf), 3.11

(s, 6 H, H₃CO), 3.33 (s, 4 H, H₂CO), 3.56 m, 4 H, thf), 6.86 (t,

³J(¹H¹H) ϭ 7.8, 1 H, p-H), 7.18 (³J(¹H¹H) ϭ 7.8, 2 H, m-H). Ϫ

δ¹³C ϭ 30.4, 31.9, 34.3, 35.3 (H₃C)₃C), 25.8, 67.8 (thf), 58.7, 72.1

(dme), 120.3, 125.3, 128.1, 136.1 (arom C). Ϫ C₂₂H₄₃O₄BMg

(406.7): calc. Mg 5.74, B 2.65, H^Ϫ0.99; found Mg 6.53, B 2.75,

H^Ϫ1.03 %.

Triethylmethylato؊magnesium tetrahydroborate diglyme (2/1), (39):

To a solution of (Et₃CO)Mg(BH₄)(OEt₂) (1.46 g, 6.36 mmol) in thf

(100 mL) was added diglyme (10 mL). The precipitate that formed

was isolated and washed with 10 mL of diethyl ether. Yield: 1.23 g

(86.9 %), m. p. 350 °C dec.. NMR (in C₆D₆): δ¹H ϭ 0.75 (t,

³J(¹H¹H) ϭ 7.5, 18 H, H₃CCH₂), 1.28 (quart, ³J(¹H¹H) ϭ 7.5,

12 H, MeCH₂), 3.11 (s, 6 H), 3.29 (s, 4 H), 3.40 (s, 4 H). Ϫ δ¹³C ϭ

60.2, 73.9 (diglyme), 15.5 (H₃CCH₂), 30.0 (MeCH₂), 60.4

(CH₂CO). Ϫ C₂₉H₅₂O_5ϪB₂Mg₂(442.86); calc. Mg 10.98, B 4.88,

H^Ϫ1.82; found Mg 11.14, B 4.76, H^Ϫ1.78 %.

(2,6-Dimethoxy)phenolato؊magnesium tetrahydroborate dimethoxy-

ethane (2/3), (34): Obtained from [(MeO)₂C₆H₃O]Mg(BH₄)(OEt₂)

(2.3 g, 8.37 mmol) and dme (50 mL). Yield: 1.36 g (51 %), m. p.

>250 °C. NMR (in d₆-Me₂SO): δ¹H ϭ 3.24 (s, H₃CO), 3.42 (s,

H₂CO), 3.64, 3.73 (H₃CO), 6.17 (m, 1 H). 6.41 (d., ³J(¹H¹H) ϭ 7.8,

Neopentylato؊magnesium tetrahydroborate diglyme (3/4), (40):

(NpO)Mg(BH₄)(OEt₂) (4.00 g, 20.0 mmol) was dissolved in diethyl

ether (20 mL), and diglyme (15 mL) was added to the solution

while stirring. The precipitate that formed was isolated. Yield:

3.77 g (46.4 %), m. p. > 300 °C. NMR (in d₆-Me₂SO): δ¹H ϭ 0.92

(s, 1 H), 3.24 (s, 8 H), 3.44 (m, 4 H), 3.51 (m, 6 H). Ϫ δ¹³C ϭ 26.2,

58.0, 69.5, 71.2. Ϫ C₃₉H₁₀₁O₁₅B₃Mg₃(1219.00): calc. Mg 7.98, B

3.55, H^Ϫ1.32; found Mg 8.21, B 3.40, H^Ϫ1.20 %.

3

1 H), 6.49 (d, J(¹H¹H) ϭ 7.8, 1 H). Ϫ δ¹³C ϭ 55.6, 56.0 (CH₃O),

58.0, 71.0 (dme), 106.7, 107.3, 108.0, 148.6, 149.3 (arom C). Ϫ

C₂₈H₅₆O₁₂B₂Mg₂(654.98): calc. B 3.31, H^Ϫ1.23; found B 3.42,

H^Ϫ1.29 %.

Triphenylmethylato؊magnesium tetrahydroborate diglyme, (41):

(Ph₃CO)Mg(BH₄)(OEt₂) (3.10 g, 8.32 mmol) was dissolved in thf

(25 mL) and diglyme was then added (10 mL) while stirring. After

3 h the precipitate was isolated. Yield: 1.53 g (43.2 %), m. p. 147

Ϫ155 °C. NMR (in C₆D₆): δ¹H ϭ 2.83 (m, 14 H) diglyme), 6.75 Ϫ

7.10 (m, 9 H), 7.12 (m, 2 H), 7.53 (d, 2 H), 7.61 (d, 2 H) arom H.

Ϫ C₂₅H₃₃O₄BMg (432.66). calc. Mg 5.62, B 2.50, H^Ϫ0.93; found

Mg 5.77, B 2.39, H^Ϫ0.95 %.

Diglyme Solvates of ROMgBH₄

Prepared in analogy to the dme solvates.

Methylato؊magnesium tetrahydroborate diglyme (2/3), (35):

(MeO)Mg(BH₄)(OEt₂) (1.52 g, 10.5 mmol) was dissolved in thf

(80 mL). To the stirred solution was added drop wise diglyme

(30 mL). About 70 % of the solvents were then removed at 60 °C

in an oil pump vacuo. The white solid that formed was isolated.

Yield 1.02 g (35.8 %)., m. p. 350 °C dec.. NMR (in C₆D₆): δ¹H ϭ

0.28, 0.93, 1.35 (all br, 40 H, diglyme, ), 3.11 ppm (9 H, H₃COMg).

Ϫ δ¹³C ϭ 30.3 (H₃CO) and other signals in the range of 55 to

73 ppm. Ϫ C₂₀H₅₂O₁₁B₂Mg₂(542.92): calc. Mg 8.95, B 3.98, H^Ϫ

1.49; found Mg 13.86, B 4.27, H^Ϫ1.33 %.

(2,6-Diisopropyl)phenolato؊magnesium tetrahydroborate diglyme

(3/1), (42): To

a stirred diethyl ether solution of (2,6-

iPr₂C₆H₃O)Mg(BH₄)(OEt₂) (45 mL, 0.3 M, 13.5 mmol) was added

diglyme (10 mL). A precipitate formed which was isolated. Yield:

1.53 g (43.2 %), m. p. 146 Ϫ 149 °C. NMR (in C₆D₆): δ¹H ϭ 1.14

3

(d, J(¹H¹H) ϭ 6.8, 12 H, (H₃C)CH), 3.32 (m, 2 H, Me₂CH), 3.24

(s, 8 H), 3.43 (m, 5H), 3.50 (m, 5 H), 6,88 (s, br, 1 H), 6.96 (m,

2 H). Ϫ δ¹³C ϭ 23.1 (H₃C)₂CH), 26.1 (Me₂CH), 58.1, 69.1, 71.3

(diglyme), 120.0, 122.8, 135.5 (arom C). Ϫ C₄₂H₅₆O₆B₃Mg₃

(762.25): calc. Mg 9.32, B 4.25, H^Ϫ1.55; found Mg 9.30, B 4.70,

H^Ϫ1.53 %.

Ethylato؊magnesium tetrahydroborate diglyme, (36): To a suspen-

sion of (EtO)Mg(BH₄)(OEt₂) in thf (1.65 g, 10.4 mmol, 20 mL) was

added diglyme (5 mL). Heating to reflux for 24 h did not dissolve

the Mg compound. The solid was isolated and proved to be com-

pound 36. Yield 1.63 g (56.6 %), m. p. 143 Ϫ 147 °C. NMR (in d₆-

Me₂SO): δ¹H ϭ 1.04 (t, ¹J(¹H¹H) ϭ 7.0, H₃CCH₂O), 1.75 (m, 1 H),

3.31Ϫ3.59 (m, 7 H). 4.29 (s, 1 H). Ϫ δ¹³C ϭ 18.5 (H₃CCH₂), 56.0

(MeCH₂O, 56.3, 69.5, 71.2 (diglyme C). ). Ϫ C₈H₂₄Og (218.39):

calc. Mg 11.13, B 4.95, H^Ϫ1.85; found Mg 10.76, B 4.65, H^Ϫ

1.73 %.

2,6-Di(tert-butyl)phenolato؊magnesium tetrahydroborate diglyme,

(43): Prepared from a solution of (2,6-tBu₂C₆H₃O)Mg(BH₄)(OEt₂)

(2.75 g, 7.20 mmol) in thf (140 mL) by adding diglyme (5 mL).

Yield of the precipitate: 2.01 g (76.4 %), m. p. 213 Ϫ 215 °C. NMR

(in C₆D₆): δ¹H ϭ 1.34 (s, 18 H, (H₃C)₃C), 3.10 (s. 6 H, Me), 3.30

(m, 4 H), 3.40 (m, 4 H), 6.85 (m, 1 H), 7.17 (d, ³J(¹H¹H) ϭ 8.1,

1 H), 7.49 (d, ³J(¹H¹H) ϭ 7.8, 1 H) (arom H). Ϫ C₂₀H₃₉O₄BMg

(378.62): calc. Mg 6.41, B 2.85, H^Ϫ1.06; found Mg 7.40, B 3.14,

H^Ϫ1.15 %.

Isopropylato؊magnesium tetrahydroborate diglyme (1/2), (37): To

(iPrO)Mg(BH₄)(OEt₂) (3.30 g, 19.1 mmol) was added diglyme

(30 mL) and the mixture refluxed for 3 h while stirring. After coo-

ling the solid was isolated. Yield 1.11 g (15.8 %), m. p. > 350 °C,

dec.. NMR (in C₆D₆): (only main signals) δ¹H ϭ 3.44 (m), 3.35

(m), 3.11(s) (diglyme), 1.10 (d, (CH₃)₂C), 1.17 (d, (H₃C)₂C), 1.27

(m, br), 4.43 (sept). 4.45 (sept). Ϫ δ¹³C ϭ 58.7, 70.9, 72.3 (diglyme),

signals of iPrO were weak.). Ϫ C₁₅H₃₉O₇BMg (366.59): calc. Mg

6.63, B 2.95, H^Ϫ1.10; found Mg 8.94, B 3.15, H^Ϫ1.09 %.

2,6-Dimethoxyphenolato؊magnesium tetrahydroborate diglyme (3/

1), (44): 2,6-[(MeO)₂C₆H₃O]Mg(BH₄)(OEt₂) (2.70 g, 10.1 mmol)

was suspended in diglyme (20 mL). After stirring for 3 h the insolu-

ble material was isolated. Yield 2.08 g (86.7 %), m. p. 290 °C dec..

NMR (in d₆-Me₂SO): δ¹H ϭ 3.24 (6 H). 3.43 (m, 4 H), 3.49 (m.

Z. Anorg. Allg. Chem. 2005, 631, 683Ϫ697

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M. Bremer, G. Linti, H. Nöth, M. Thomann-Albach, G. E. W. J. Wagner

Table 4 Crystallographic data of compounds 3 to and 46 and data related to data collection and structure solution

Compound

3

4

20

6

21

33

49

50

Chem. formula

Form. wght.

C₇H₂₁O₂BMg C₈H₂₃O₂BMg C₁₁H₂₇O₂BMg C₉H₂₅O₂BMg C₁₄H₂₅O₃BMg C₂₂H₄₃O₄BMg C₁₂H₃₆O₃NBMgSi₂C₇H₂₃O₂BMgSi

172.4

186.4

226.4

200.4

276.5

406.7

333.7

202.5

Cryst. Size, mm

Cryst. system

Space group

0.3x0.5x0.55

monoclinic

P2₁/n

9.379(2)

10.531(2)

11.668(3)

0.6x0.3x0.3

monoclinic

P2₁/n

9.754(2)

10.960(2)

11.720(2)

0.55x0.45x0.32 0.48x0.3x0.25

0.6x0.6x0.45

monoclinic

P2₁/c

7.987(3)

10.087(2)

19.817(4)

0.5x0.5x1.3

monoclinic

P2₁/n

12.198(1)

14.090(1)

14.882(1)

0.4x0.28x0.31

monoclinic

P2₁/m

9.143(4)

12.681(5)

10.231(5)

0.4x0.4x0.45

monoclinic

P2₁/n

9.412(2)

13,888(3)

10.517(2)

triclinic

P1

monoclinic

P2₁/n

¯

˚

a, A

8.622(1)

9.281(1)

9.486(1)

81.89(1)

80.71(1)

81.65(1)

735.8(1)

2

9.365(7)

13.192(8)

11.528(9)

˚

b, A

˚

c, A

α, °

β, °

γ, °

95.87(1)

91.23(1)

94.47(6)

92.04(3)

105.56(6)

115.72(1)

93.89(7)

3

˚

V, A

1146.4(4)

4

0.999

0.115

384

1252.5(4)

4

0.988

0.105

416

1422(1)

4

1595.5(8)

4

1.151

0.106

600

2464(1)

4

1.096

0.089

896

1067(1)

2

1.037

0.200

368

1371.6(7)

4

0.980

0.187

448

Z

ρ(calcd.), Mg/m³

µ, mm^Ϫ1

F(000)

1.022

0.936

0.098

0.095

252

448

Index range

Ϫ1ՅhՅ10

0ՅkՅ12

Ϫ13ՅlՅ13

50.0

203

2160

0ՅhՅ11

0ՅkՅ13

Ϫ13ՅlՅ13

50

193

2347

0ՅhՅ10

Ϫ11ՅkՅ11

Ϫ11ՅlՅ11

52

293

3235

Ϫ4ՅhՅ10

Ϫ1ՅkՅ14

Ϫ12ՅlՅ12

45.0

293

2091

Ϫ2ՅhՅ8

Ϫ9ՅkՅ11

Ϫ22ՅlՅ22

47.0

193(3)

2843

0ՅhՅ13

0ՅkՅ15

Ϫ16ՅlՅ16

47.0

193

3946

0ՅhՅ10

0ՅkՅ14

Ϫ12ՅlՅ10

50.0

253

1994

0ՅhՅ11

0ՅkՅ17

Ϫ12ՅjՅ12

52.0

273

2716

2619

1805

2.95

2 θ, °

Temp, K

Refl. collected

Refl. unique

Refl. observed (4σ)

R (int.)

1779

1443

2094

2897

1864

1383

2309

3554

1893

1712 (3σ)

0.0202

155

2420 (4σ)

0.0221

148

1674 (3σ)

0.0885

199

2781 (3σ)

0.0471

358

1193 (4σ)

0.0248

110

0.0463

112

0.0017

118

No. variables

109

Weight. scheme

σ²/0.0017

σ²/0.0003

σ²/0.00012

σ²/0.0018

σ²/0.0001

σ²/0.0003

σ²/0.0017

a)

GOOF

1.25

2.40

3.72

2.01

1.37

2.20

2.29

1.39

Final R (4σ)

Final wR2

Larg. res. peak e/A

0.0473

0.0659

0.35

0.0625

0.0742

0.49

0.0679

0.079c2

0.47

0.0697

0.0948

0.39

0.0638

0.1497

0.492

0.065

0.0635(wR)

0.30

0.0721

0.913

0.036

0.0569

0.0799

0.32

3

˚

^a)w^Ϫ1ϭ δ²(F) ϩ x(F²)

4 H) diglyme; 3.63, 3.73 (each s, 18 H, OCH₃), 6.41 (d, 6 H), 6.49

(t, ³J(¹H¹H) ϭ 7.8, 3 H). Ϫ C₃₀H₅₀O₁₂B₃Mg₃(711.57): calc. B 4.56,

H^Ϫ1.70; found B 4.56, H^Ϫ1.73 %.

Bis(trimethylsilyl)amino-magnesium tetrahydroborate-bis(tetrahy-

drofuran), (47): 46 (3.68 g, 13.5 mmol) was dissolved in thf (40 mL).

The solution was then reduced in vacuo to half of its volume. Then

pentane was allowed to diffuse into the solution. After 6 d crystals

had separated. These were dried shortly in vacuo. They rapidly hy-

drolysed in air with formation of a colorless powder. Yield: 3.34 g

(87.8 %), m. p. 61 Ϫ 64 °C. NMR (in C₆D₆): δ¹H ϭ0.41 (s, Me₃Si),

Bis(2-isopropoxyphenolato)-magnesium-dimethylsulfoxide,

(45):

Compound 13 (0.7 g, 2.2 mmol) was crystallized from Me₂SO

(10 mL) . This produced crystals of compound 45. Yield was not

determined. m. p. > 300 °C. NMR (in d₆-Me₂SO): δ¹H ϭ 1.35 (d,

³J(¹H¹H) ϭ 5.7, 6 H, CH(CH)₃), 2,71 (s, 6 H, Me₂SO). 4.70 (m,

1 H, OCHMe₂), 6-84 Ϫ 6.86 (br, m, 4 H).δ¹³C ϭ 21.2 (CH(CH₃)₂,

69.7 (CHMe₂), 117.0, 117.7, 121.3, 122.0, 147.1 (arom. C).

C₂₀H₂₈O₅MgS (404.81): calc. C 59.34, H 6.97, O 19.76; found C

56.18, H 6.79, O 19.59 %.

1.29, 3.71 (thf).

Ϫ Ϫ

δ¹³C: 6.2 (Me₃Si), 25.2, 69.6 (thf).

C₁₄H₃₈O₂NBMgSi₂(434.74): calc. Mg 7.07, B 3.15, H^Ϫ1.17; found

Mg 6.92, B 3.10.02, H^Ϫ1.08 %.

Bis(trimethylsilylamino)-magnesium tetrahydroborate dimethoxy-

ethane, 48: To 2.37 g of 46 (8.7 mmol) was added dme (30 mL).

The solution was stirred for one h, then most of the volatile parts

were removed in vacuo (oil pump). Pentane was allowed to diffuse

into the remaining solution within a period of 5 d. Colorless need-

les separated within this time. They were isolated and shortly dried

in vacuo. Yield: 2.14 g (85.2 %), m. p. 63 Ϫ 65 °C. NMR (in C₆D₆):

δ¹H ϭ 0.01 (Me₃Si), 3.19 (H₃CO), 3.40 (t, CH₂O). Ϫ δ¹³C ϭ 2.7

Bis(trimethylsilyl)amino magnesium tetrahydroborate diethyl ether,

(46): A solution of (Me₃Si)₂NH (2.13 mL, 18.6 mmol) in diethyl

ether (20 mL) was dropped into a stirred solution of EtMg(BH₄)

in diethyl ether (0.93 M, 20 mL). After gas evolution had subsided

the ether was stripped off from the solution in vacuo. No crystals

separated on cooling the solution after various steps of ether remo-

val. Finally a very viscous oil remained which hydrolysed rapidly

in air. However, it turned solid after standing for 3 days. Yield:

4.5 g (89 %); m. p. 53 Ϫ 55 °C. NMR (in C₆D₆): δ¹H ϭ Ϫ0.37

(quart. ¹(J¹¹B¹H) ϭ 82.3, BH₄), 0.41 (s, 18 H, SiMe₃), 1.27 (t, 6 H,

CH₂CH₃)), 13.49 (quart, 4 H, CH₂). Ϫ ¹³C ϭ 6.1 (SiMe), 14.4, 65.0

(Me₃Si), 57.3 (H₃CO), 71.1 (CH₂O).

Ϫ C₁₀H₃₂O₂NBMgSi₂

(289.67): calc. Mg 7.07, B 3.15, H^Ϫ1.17; found Mg 6.92, B 3.02,

H^Ϫ1.08 %.

Bis(trimethylsilyl)aminomagnesium tetrahydroborate-diglyme, (49):

0 1.63 g of 46 (5 mmol) were dissolved in diglyme (30 mL). In va-

cuo about 10 mL of the solvents were removed, and the remaining

solution stored at Ϫ18 °C for 5 d. Colorless crystals of single cry-

stal quality formed. Yield: 1.5 g (82 %), m. p. 68 Ϫ 71 °C. NMR

(Et₂O).

Ϫ ϭ Ϫ37.8 (broad). Ϫ

δ¹¹B (in diethyl ether)

C₁₀H₃₂NBMgSi₂(273.67): calc. Mg 8.86, B 3.95, H^Ϫ1.47; found

Mg 8.54, B 3.84, H^Ϫ1.30 %.

696

zaac.wiley-vch.de

Z. Anorg. Allg. Chem. 2005, 631, 683Ϫ697

Metal Tetrahydroborates and Tetrahydroborato Metalates

(in C₆D₆): δ¹H ϭ 0.11, 0.39 (s, ratio 2:1, Me₃Si), 3.10 (H₃CO).

3.41 (MeOCH₂), 3.51 (CH₂CH₂O). Ϫ δ¹³C ϭ 2.6, 2.9 (Me₃Si), 58.7

(CH₃O), 70.9 (MeOCH₂O), 72.3 (CH₂CH₂O). C₁₂H₃₆O₃NBMgSi₂

(333.72): calc. Mg 7.29, B 3.24, H^Ϫ1.20; found Mg 7.21, B 3.17,

H^Ϫ1.13 %.

[2] S. Marynick, W. N. Lipscomb, J. Am. Chem. Soc. 1973, 95,

7244.

[3] T. H. Cook, G. L. Morgan, J. Am. Chem. Soc. 1969, 91, 774.

J. W. Nibler, J. Am. Chem. Soc. 1972, 94, 3349.

[4] P. Wolfgardt, diploma thesis, University of Munich, 1992.

[5] J. F. Stanton, W. N. Lipscomb, P. J. Bartlett, J. Chem. Phys.

1988, 9, 5726, and lit. cited therein. R. Ahlrichs, Chem. Phys.

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[7] H. Nöth, Z. Naturforsch. 1982, 37b, 1499.

[8] E. B. Lobkowskii J. W. Titov, C. B. Pricha, M. Ya. Antipin,

Yu. T. Stuchkov, Zh. Strukt. Khim. 1982, 123, 23.

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1988, 17, 769.

[10] E. C. Ashby, A. B. Goel, Inorg. Chem. 1977, 16, 2082.

[11] E. C. Ashby, A. B. Goel, Inorg. Chem. 1977, 16, 2941.

[12] J. E. Ewerling, H. Nöth, Z. Naturforsch. 1970, 25b, 785.

[13] P. Strebel, Ph. D. Thesis, University of Munich, 1958.

[14] P. Strebel, E. Wiberg, Liebigs Ann. Chem. 1957, 607, 9.

[15] N. Edelstein, Inorg. Chem. 1981, 20, 297.

Trimethylsiloxomagnesium tetrahydroborate diethyl ether, (50): A

solution of 46 (1.14 g, 4.0 mmol) in diethyl ether (10 mL) was kept

for a week in a refrigerator at Ϫ25 °C. Within this time colorless,

regularly shaped crystals separated. These were isolated, dried in a

stream of N₂and analyzed by X-ray crystallography. The crystals

proved to be compound 50. The same compound was obtained

after several recrystallisations of 46 from diethyl ether. Yield: 0.52 g

(65.1 %), m.p. 71Ϫ73 °C. NMR (in C₆D₆): δ¹H ϭ Ϫ0.39 (quart.,

¹J(¹¹B¹H) ϭ 82.4 Hz, BH₄), 0.31 (s, Me₃Si), 0.89, 3.52 Et₂O). Ϫ

δ¹³C: 3.4 (Me₃Si), 13.9, 65.6 (Et₂O). Ϫ δ¹¹B (in THF): Ϫ39.9

(quint., ¹J(¹¹B¹H) ϭ 81.4. Ϫ C₇H₂₃O₂BMgSi (202.46): calc. C

41.53, B 5.34, Mg 12.02; H^Ϫ11.45 found C 38.72, B 5.47, Mg

12.25. H^Ϫ11.02 %.

[16] T. J. Marks, J. R. Kolb, Chem. Rev. 1977, 77, 263.

[17] M. G. H. Wallbridge, Progr. Inorg. Chem. 1970, 11, 99.

[18] H. Beall, C. M. Bushweller, Chem. Rev. 1973, 73, 465.

[19] H. Nöth, B. Wrackmeier, Basic Principles and Progress, NMR

Spectroscopy of Boron Compounds, Editors, P. Diehl, E. Fluck,

P. Kosfeld, Springer Publishers, Berlin, Heidelberg, New

York, 1978.

[20] G. W. J. Wagner, PhD Thesis, University of Munich, 1994.

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[23] V. Gutmann, The Donor Acceptor Approach to Molecular In-

teractions, Plenum Press, New York, London, 1978.

[24] D. C. Bradley, M. B. Hursthouse, A. C. Sullivan, Polyhedron

1990, 9, 29.

X-ray Structure analysis

Reflection data were collected on a Siemens P4 diffractometer ope-

rated with graphite monochromatized MoKα radiation in the ω/2θ

mode using a scintillation counter for intensity measurements. The

unit cell and the orientation matrix were determined by centering

the reflection found in rotation photos. Scan time varied from 5 to

30 s depending on intensity. After data reduction the structures

were solved with the program packages SHELXPLUS and SHELX

93 [29]. Relevant crystallographic data as well as information on

the structure solution are summarized in Table 4. Additional cry-

stallographic data are deposited with the Cambridge Crystallogra-

phic Data Centre as publication numbers CCD-256049Ϫ256056

and can be obtained free of charge on application to CCDC, 12

Union Road, Cambridge CB2 1EZ, UK Fax ϩ44-1223-336033; e-

mail: deposit@ccdc.cam.as.uk. In figures 1 Ϫ 7 all thermal ellip-

soids are depicted on a 25 % probability scale. Bond lengths are

[25] A. M. Domingos, G. W. Sheldrick, Acta Crystallogr. 1973,

B30, 517.

[26] J. C. Calabrese, M. A. Cushing, S. D. Mel, Inorg. Chem. 1988,

27, 867.

˚

given in A, and bond angles in degrees. Estimated standard devia-

tions are set in parenthesis.

[27] T. J. Boyle, E. N. Coker, M. A. Rodriguez, R. A. Kemp, M.

Z. Maller, Chem. Matr. 2003, 15, 309.

[28] C. A. Zechmann, T. J. Boyle, M. A. Rodriguez, R. A. Kemp,

Polyhedron 2000, 19, 2557.

References

[1] Part 29, H. Nöth, M. Warchhold, Eur. J. Inorg. Chem. 2003,

111.

[29] G. W. Sheldrick, University of Göttingen, 1993.

Z. Anorg. Allg. Chem. 2005, 631, 683Ϫ697

zaac.wiley-vch.de

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Article Doi

Metal tetrahydroborates and tetrahydroborato metalates. 30 [1] Solvates of alcoholato-, phenolato-, and bis(trimethylsilyl)amido-magnesium tetrahydroborates XMgBH4(Ln)

DOI: 10.1002/zaac.200400420

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