A novel diaminoborate ligand system derived from 1,8-diaminonaphthalene and 9-BBN: Preparation of titanium and zirconium complexes and crystal structure of the titanium complex

Article abstract of DOI:10.1039/a607194e

1,8-Diaminonaphthalene reacts selectively with 1 equiv. of 9-BBN to form a bridging amine-aminoborane, and with 2 equiv. of 9-BBN to form a non-bridging bis(aminoborane); these aminoboranes give rise to a novel diaminoborate (4e,1-) ligand system by reactions with dimethylamides of titanium and zirconium.

Full text of DOI:10.1039/a607194e

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A novel diaminoborate ligand system derived from 1,8-diaminonaphthalene
and 9-BBN: preparation of titanium and zirconium complexes and crystal
structure of the titanium complex
Galia Bar-Haim, Romem Shach and Moshe Kol*†
School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel
1,8-Diaminonaphthalene reacts selectively with 1 equiv. of
9-BBN to form a bridging amine–aminoborane, and with 2
equiv. of 9-BBN to form a non-bridging bis(aminoborane);
these aminoboranes give rise to a novel diaminoborate
(4e,12) ligand system by reactions with dimethylamides of
titanium and zirconium.
the two faces of the naphthalene ring, while the three N–H
protons rapidly equilibrate between the two nitrogen atoms.
On the other hand, the two boron atoms in L2 are three-
coordinate according to 11B NMR spectroscopy, i.e. they are not
bridging between the two nitrogen atoms. Three peaks for six
aromatic protons indicate a symmetrical naphthalene system.
Two peaks for four bridgehead carbons of the two 9-BBN units
indicate a high barrier to rotation of the 9-BBN groups around
the B–N bonds attributed to pp–pp interactions.§
The search for new ligand systems, and especially ones that
could replace the (6e,12) cyclopentadienyl type ligands, is a
major area of research in current inorganic and organometallic
chemistry. Most progress has been achieved with ligands based
on nitrogen donors like trispyrazolylborate,1 benzamidinate,2
silyl amide,3 boryl amide4 and others.5 A major aspect in
designing of such ligands, other than their electronic character,
is their steric effects on the metal environment. In this report we
describe a novel boron nitrogen ligand derived via two routes
from 1,8-diaminonaphthalene and 9-BBN, and its complexes
with titanium and zirconium.
Aniline was reported to react with 9-borabicyclo[3.3.1]-
nonane (9-BBN) to form the corresponding B-phenylamino-
9-BBN with evolution of hydrogen gas.6 We found that
1,8-diaminonaphthalene can react with 9-BBN, with evolution
of hydrogen gas, to form two isolable products, depending on
the molar ratio of the reactants: addition of exactly 1 equiv. of
9-BBN to 1,8-diaminonaphthalene results in highly selective
formation of an amine–aminoborane, HL1, whereas addition of
2 equiv. of 9-BBN to 1,8-diaminonaphthalene yields a bis-
(aminoborane), L2, quantitatively. Adding 1 equiv. of 9-BBN to
isolated HL1 produces L2 in high yield as well (Scheme 1).
When stored and handled under dry nitrogen at room tem-
perature, HL1 and L2 are stable for months.
Interestingly, related aromatic diamines do not exhibit the
same high selectivity in formation of an amine–aminoborane.
Thus, reaction between 1,2-phenylenediamine and 2 equiv. of
9-BBN gave a bis(aminoborane), L3, quantitatively, whose
spectral properties are analogous to those of L2 (Scheme 2).¶
However, reaction between 1,2-phenylenediamine and 1 equiv.
of 9-BBN gave an inseparable mixture of products.
HL1 reacted cleanly with tetrakis(dimethylamino)titanium in
an amine elimination reaction giving [TiL1(NMe2)3] 1 and
dimethylamine. 1 was isolated in high yield as an orange
crystalline solid upon evaporation of the solvent and recrys-
tallisation from diethyl ether–pentane at 235 °C. Tetrakis-
(dimethylamino)zirconium
reacted
similarly
yielding
[ZrL1(NMe2)3] 2 as a white crystalline solid after a similar
work-up (Scheme 3, route a). 1 and 2 exhibit almost identical
NMR spectra that indicate that, in solution, L1 is a (4e,12)
symmetrical chelating diaminoborate-type ligand.∑
The X-ray crystal structure of the titanium complex 1 showed
that the chelating diaminoborate ligand effectively blocks one
face of the metal (Fig. 1).** The main structural feature is an
almost perfectly symmetrical Ti–N–B–N four-membered ring,
having B–N and Ti–N bond lengths of 1.60 and 2.29 Å,
respectively, and N–B–N and N–Ti–N bond angles of 94.4 and
61.4°, respectively.
1H, 13C and 11B NMR indicate that HL1 is pseudo-C2v-
symmetrical on the NMR timescale, with the boron atom
bridging between the two nitrogen atoms.‡ The three N–H
protons appear as a sharp singlet in the 1H NMR spectrum even
at 250 °C. These data are consistent with a fast dynamic
process in which the bridging 9-BBN unit is rocking between
H
H
B
B
H2N
NH2
N
N
2 equiv. 9-BBN, -2 H2
NH2 NH2
L3
Scheme 2
1,8-diaminonaphthalene
1 equiv. 9-BBN, -H2
2 equiv. 9-BBN, -2 H2
(Me2N)3M
B
N
N
B
H H
B
H
H
B
H
H
M(NMe2)4
M(NMe2)4
1 equiv. 9-BBN, -H2
N
N
N
N
HL1
L2
H
-HNMe2
route a
-BBN-NMe2
route b
1 M = Ti
2 M = Zr
HL1
L2
Scheme 1
Scheme 3
Chem. Commun., 1997
229

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light yellow solid that precipitated was washed with ether and dried to give
1
pure HL1 in ca. 95% yield: H NMR (C6D6, 200 MHz) d 0.58 (br s, 2 H,
CH), 1.60–2.20 (m, 12 H, 6 CH2), 4.06 (s, 3 H, NH), 6.12 (d, 2 H, J 8 Hz),
7.08 (t, 2 H, J 8 Hz), 7.24 (d, 2 H, J 8 Hz). 13C{1H} NMR ([2H8]thf, 50.4
MHz) d 24.4 (CH), 27.3 (CH2), 33.2 (CH2), 113.4 (CH), 120.6 (CH), 128.3
(CH). 11B NMR (C6D6, 115.5 MHz) d 21.7.
N(24)
N(25)
Ti(1)
§ L2 was prepared on a 10 mmol scale, by adding 2 equiv. of 9-BBN in
hexanes to a solution of 1,8-diaminonaphthalene in ether under nitrogen,
and stirring at room temp. until hydrogen evolution ceased (ca. 30 min).
Removing the solvents under vacuum yielded white crystalline L2
quantitatively: 1H NMR (C6D6, 200 MHz) d 1.29–1.95 (m, 14 H), 6.92 (s,
2 H, NH), 7.04 (d, 2 H, J 8 Hz), 7.15 (t, 2 H, J 8 Hz), 7.44 (d, 2 H, J 8 Hz).
13C{1H} NMR (C6D6 50.4 MHz) d 22.9 (CH), 28.1 (CH), 24.4 (CH2), 33.8
(CH2), 34.4 (CH2), 126.2 (CH), 123.6 (CH), 126.3 (CH). 11B NMR (C6D6,
115.5 MHz) d 49.9.
B(3)
N(2)
N(4)
N(23)
¶ L3: 1H NMR (C6D6, 200 MHz) d 1.30–2.04 (m, 14 H), 5.67 (s, 2 H, NH),
7.02 (m, 2 H), 7.23 (m, 2 H); 13C{1H} NMR (C6D6, 50.4 MHz) d 23.2, 28.0
(CH), 24.5, 34.0, 34.8 (CH2), 125.3, 125.9 (CH); 11B NMR (C6D6, 115.5
MHz) d 51.6.
∑ [TiL1(NMe2)3] 1: 1H NMR (C6D6, 200 MHz) d 0.63 (br s, 1 H, CH), 0.67
(br s, 1 H, CH), 1.75–2.20 (m, 12 H, 6 CH2), 2.84 (s, 18 H), 4.11 (s, 2 H,
NH), 6.39 (d, 2 H, J 7 Hz), 7.13 (t, 2 H, J 7 Hz), 7.20 (d, 2 H, J 7 Hz). 11
B
NMR (C6D6, 115.5 MHz) d 22.8. [ZrL1(NMe2)3] 2: 1H NMR (C6D6, 200
MHz) d 0.69 (br s, 2 H, CH), 1.68–2.11 (m, 12 H, 6 CH2), 2.69 (s, 18 H) 3.99
(s, 2 H, NH), 6.37 (d, 2 H, J 7 Hz), 7.08 (t, 2 H, J 7 Hz), 7.22 (d, 2 H, J 7
Hz). 11B NMR (C6D6, 115.5 MHz) d 24.4.
Fig. 1 Molecular structure of [TiL1(NMe2)3] 1. Hydrogen atoms omitted for
clarity. Selected distances (Å) and angles (°): Ti(1)–N(2) 2.302(3), Ti(1)–
N(4) 2.282(4), Ti(1)–N(23) 1.874(3), Ti(1)–N(24) 1.927(5), Ti(1)–N(25)
1.912(3), B(3)–N(2) 1.586(7), B(3)–N(4) 1.604(5); N(2)–Ti(1)–N(4)
61.4(1), N(2)–B(3)–N(4) 94.4(4), Ti(1)–N(2)–B(3) 88.0(2), Ti(1)–N(4)–
B(3) 88.3(3).
** Crystal data for C24H40BN5Ti, M = 457.32, monoclinic, space group
P21/a, a = 15.649(4), b = 18.949(2), c = 18.404(3) Å, b = 111.72(2)°,
U = 5070(2) Å3, Z = 8, Dc = 1.198 g cm23, l(Mo-Ka) 0.71073 Å,
2qmax
= 45.96°. Data were collected on an Enraf-Nonius CAD4
Several reports describing systems that include M–N–B–N
four-membered rings have appeared (M = Ge, Sn, Ti, Zr, Cr).7
However, in those cases the N–B–N unit either forms a (4e,22)
ligand, or it is a part of a larger heterocyclic system. The system
we describe herein is the first case of a (4e,12) diaminoborate
ligand system, to the best of our knowledge. Unlike other
(4e,12) ligands, such as benzamidinate or acetylacetonate, the
diaminoborate ligand consists of tetrahedral atoms rather than
trigonal-planar atoms close to the metal centre. Since tetra-
hedral atoms are inherently more sterically demanding than
trigonal-planar atoms, close control of the metal environment
can be achieved by proper substitutions in this ligand system.
The N,NA-bis(trimethylsilyl) derivative of 1,8-diamino-
naphthalene acts as a (4e,22) ligand.8 Considering the B–Si
diagonal relationship one may expect that L2 would act as a
chelating (4e,22) bis(borylamide) ligand4 by double deproto-
nation. However, this was not the case. Instead, L2 reacted
cleanly with tetrakis(dimethylamino)-titanium and -zirconium
to yield the diaminoborate complexes 1 and 2, respectively,
accompanied by 9-BBN–NMe2 as a by-product9 (Scheme 3,
route b). The driving force for this unprecedented trans-
formation is, apparently, the stability of the boron-bridged
structure, in the 1,8-diaminonaphthalene skeleton.
diffractometer using an orange crystal mounted in a sealed glass capillary at
room temperature. Of a total of 6245 collected reflections, 6027 were
unique (Rint = 0.0387). The structure was solved by direct methods. The
final cycle of full-matrix least-squares refinement was based on 6027
observed reflections (I > 0) and 571 variable parameters and converged
with unweighted and weighted agreement factors of R
Rw = 0.1391 for the entire data. GOF = 1.010. Residual electron densities:
+0.20, 20.29 e Å23
= 0.0995,
.
Atomic coordinates, bond lengths and angles, and thermal parameters
have been deposited at the Cambridge Crystallographic Data Centre
(CCDC). See Information for Authors, Issue No. 1. Any request to the
CCDC for this material should quote the full literature citation and the
reference number 182/314.
References
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in 1,8-disubstituted naphthalenes.10 We think that the selective
formation of HL1 and of the (4e,12) metal complexes of L1 (1
and 2) are a result of such effects. We are currently investigating
these effects as well as the reactivity of the novel diaminoborate
complexes.
We thank the Israel Science Foundation administered by the
Israel Academy of Sciences and Humanities for financial
support.
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Footnotes
10 H. A. Staab and T. Saupe, Angew. Chem., Int. Ed. Engl., 1988, 27,
865.
† E-mail: moshekol@ccsg.tau.ac.il
‡ HL1 was prepared on a 20 mmol scale by adding 1 equiv. of 9-BBN in
hexanes to a solution of 1,8-diaminonaphthalene in ether under nitrogen,
and stirring at room temp. until hydrogen evolution ceased (ca. 30 min). The
Received, 22nd October 1996; Com. 6/07194E
230
Chem. Commun., 1997