New binucleating ligands to support dizirconium organometallics

Article abstract of DOI:10.1039/b106972a

A new class of bis(amidinato) ligands, which feature dibenzofuran or 9,9-dimethylxanthene spacers, are used to prepare structurally characterized dizirconium complexes.

Full text of DOI:10.1039/b106972a

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New binucleating ligands to support dizirconium organometallics
John R. Hagadorn
Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215, USA.
E-mail: hagadorn@stripe.colorado.edu
Received (in West Lafayette, IN, USA) 2nd August 2001, Accepted 1st September 2001
First published as an Advance Article on the web 2nd October 2001
A new class of bis(amidinato) ligands, which feature
dibenzofuran or 9,9-dimethylxanthene spacers, are used to
prepare structurally characterized dizirconium complexes.
The development of single-site catalysts for selective bond
forming and cleaving processes is a major topic in organo-
metallic chemistry. Toward this goal, much effort has gone into
controlling chemical reactivity through the modification of
supporting ligands.1 This approach has been highly successful
for the preparation of new mononuclear catalysts; however,
equivalent advances have not been made for bimetallic
complexes. The use of well-defined bimetallics as catalysts,
however, is highly desirable because multiple metals acting
cooperatively can activate substrates and perform reactions that
fail to occur in mononuclear systems.2,3 Thus, we have begun to
develop new binucleating ligands which will allow us to explore
the reactivity of binuclear organometallic complexes. Here we
report two new bis(amidinato)4 ligands and their structurally
characterized Zr derivatives.
Fig. 1 Molecular structure of (Pri4DBF)Zr2(CH2Ph)6·1.5Et2O drawn with
50% thermal ellipsoids. Half occupancy ether is omitted. Selected distances
(Å) and angles (°): Zr1–Nav 2.25, Zr1–Cav (excl. C28) 2.27, Zr1–C28
2.721(3), Zr1–C69 4.467(4), Zr2–Nav 2.25, Zr2–Cav (excl. C49) 2.28, Zr2–
C49 2.682(3), Zr2–C69 4.273(4), C48–C69, 3.756(6), Zr1–Zr2 8.574(1);
Zr1–C27–C28 90.9(2), Zr2–C48–C49 88.1(2).
nificantly longer than the value predicted from simple MM2
calculations (7.3 Å). Interestingly, this is due to the presence of
a cocrystallized Et2O that is located between the two Zr atoms
yet is not coordinated to either metal. The Zr–ether distances are
short (Zr1–C69, 4.46 Å; Zr2–C69, 4.27 Å), with the H atoms
bound to C69 approaching to within 3.5 Å of Zr2. This presents
the intriguing possibility of using the intermetal gap as a
docking site for incoming substrates that could subsequently
react with metal-bound ligands.
The bis(amidine) ligands (Pri4DBF)H2† and (Pri4Xan)H2
were prepared by the reaction of diisopropylcarbodiimide with
doubly deprotonated dibenzofuran or 9,9-dimethylxanthene.5
The ligands are highly soluble in hydrocarbon solvents, but they
can be readily isolated as colorless crystals from warm
acetonitrile solutions in moderate yield. Combustion analysis
The related xanthene-bridged dizirconium complex (Pri4-
Xan)Zr2(CH2Ph)6 was prepared following the methodology
similar to that used for (Pri4DBF)Zr2(CH2Ph)6 (Scheme 1). The
product was isolated as yellow microcrystals from toluene–
hexamethyldisiloxane mixtures in high yield. 1H NMR spectra
(C6D6) reveal a fluxional ligand environment. Crystals suitable
for X-ray diffraction studies were grown from Et2O at 240 °C.
Metrical parameters (Fig. 2) are very similar to those of (Pri4-
DBF)Zr2(CH2Ph)6; however, the xanthene-bridge closes the
gap between the two Zr(CH2Ph)3 moieties. Consequently, there
is not a solvent molecule present between them, and the
intermetal distance is reduced by over 2 Å [Zr1–Zr2 6.489(1)
Å].
1
and H NMR spectroscopy confirmed their formulations. IR
absorption spectroscopy of (Pri4Xan)H2 (Nujol) revealed char-
acteristic absorptions at nNH 3440 and nCN 1639 cm21. Related
parameters for (Pri4DBF)H2 were observed at 3434, 3427, 3402
and 1633 cm21
.
Entry into transition metal derivatives was accomplished by
reaction of (Pri4DBF)H2 with 2 equivalents of Zr(CH2Ph)4 in
toluene solution [eqn. (1)]. Removal of the volatiles under
(Pri4DBF)H2 + 2Zr(CH2Ph)4 ? (Pri4DBF)Zr2(CH2Ph)6
+ 2PhCH3 (1)
reduced pressure afforded yellow (Pri4DBF)Zr2(CH2Ph)6 in
quantitative yield. 1H and 13C NMR spectroscopy (C6D6) reveal
a symmetrical ligand environment and equivalent benzyl
ligands, consistent with fast fluxional behavior in solution.
Crystallization of the product, however, was slow, and crystals
suitable for X-ray diffraction‡ studies were obtained only after
storing Et2O solutions at 240 °C for two weeks. The solid-state
structure (Fig. 1) confirmed the formulation of the dizirconium
complex. Geometry at each Zr center resembles that of the
guanidinate derivative {CyNC[N(SiMe3)2]NCy}Zr(CH2Ph)3,
which was recently reported by Richeson and coworkers.6 The
Reaction of a single equivalent Zr(CH2Ph)4 with (Pri4Xan)H2
(Scheme 1) yielded an orange solution. After stirring overnight
and removal of the volatiles, the residue was extracted into
warm Et2O. Cooling to 240 °C gave orange crystals in good
yield. Combustion analysis of the product indicated the
2
h -benzyl ligands feature Zr–CH2–Cipso angles of 90.9(2) and
88.1(2)° with Zr–Cipso distances of 2.721(3) and 2.682(3) Å,
2
values which are similar to other reported Zr(h -benzyl)
compounds.7 The intermetal distance of 8.574(1) Å is sig-
Scheme 1
2144
Chem. Commun., 2001, 2144–2145
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b106972a

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(C6D6): d 7.52 (dd, J 1.2, 7.5 Hz, 2H), 7.25 (t, J 7.6 Hz, 12H), 7.11 (t, J 7.5
Hz, 2H), 7.06–7.00 (m, 20H), 3.27 (sept, J 6.5 Hz, 4H), 2.53 (s, 12H), 1.08
(d, J 6.5 Hz, 12H), 0.93 (d, J 6.5 Hz, 12H). 13C{1H} NMR (C6D6): d 175.9,
152.1, 143.8, 129.9, 128.5, 128.3, 126.8, 124.7, 123.7, 123.2, 121.8, 116.6,
77.2, 50.9, 25.3, 24.2. For (Pri4Xan)Zr2Bn6: 1H NMR (C6D6): d 7.18 (t, J 7.7
Hz, 12H), 7.04 (dd, J 1.6, 7.8 Hz, 2H), 7.00 (d, J 7.3 Hz, 12H), 6.97 (t, J 7.4
Hz, 6H), 6.92 (t, 7.5 Hz, 2H), 6.71 (dd, J 1.6, 7.3 Hz, 2H), 3.36 (sept, J 6.6
Hz, 4H), 2.65 (s, 12H), 1.35 (s, 6H), 1.31 (d, J 6.5 Hz, 12H), 0.91 (d, J 6.7
Hz, 12H); 13C{1H} NMR (C6D6): d 177.1, 146.2, 144.3, 130.2, 129.7,
128.6, 128.3, 128.1, 127.9, 127.5, 123.7, 123.0, 120.5, 78.2, 51.0, 34.2,
33.4, 26.4, 24.2. For (Pri4Xan)ZrBn2: 1H NMR (C6D6): d 7.61 (d, J 7.6 Hz,
4H), 7.42 (t, J 7.6 Hz, 4H), 7.08 (t, J 7.3 Hz, 2H), 7.05 (dd, J 1.4 Hz, 7.3 Hz,
2H), 6.93 (dd, J 1.4, 7.9 Hz, 2H), 6.74 (t, J 7.6 Hz, 2H), 3.54 (sept, J 6.5 Hz,
4H), 2.92 (s, 4H), 1.27 (s, 6H), 1.12 (d, J 6.5 Hz, 12H), 0.84 (d, J 6.3 Hz,
12H); 13C{1H} NMR (C6D6): d 165.2, 148.5, 144.5, 129.1, 128.5, 128.3,
127.9, 126.7, 126.5, 125.1, 123.0, 121.6, 75.2, 50.8, 34.2, 32.5, 25.6,
25.1.
‡ Crystal data for (Pri4DBF)Zr2(CH2Ph)6·1.5Et2O: C74H91N4O2.5Zr2, M =
¯
1258.95, triclinic, space group P1 (no. 2), a = 11.392(2), b = 18.274(4), c
Fig. 2 Molecular structure of (Pri4Xan)Zr2(CH2Ph)6·Et2O drawn with 50%
thermal ellipsoids. Cocrystallized ether is omitted. Selected distances (Å)
and angles (°): Zr1–Nav 2.24, Zr1–Cav (excl. C31) 2.28, Zr1–C31 2.627(3),
Zr2–Nav 2.24, Zr2–Cav (excl. C52) 2.28, Zr2–C52 2.678(4), Zr1–Zr2
6.489(1); Zr1–C30–C31 86.2(2), Zr2–C51–C52 89.8(2).
= 18.717(4) Å, a = 109.66(3), b = 102.65(3), g = 105.50(3)°, V =
3324.7(12) Å3, Z = 2, m = 0.34 mm21, T = 2138 °C, 20243 independent
reflections, Rint = 0.0482, 11705 observations, 783 parameters, R1
=
0.0603, wR2 = 0.1571, GOF = 0.973. For (Pri4Xan)Zr2(CH2Ph)6·Et2O:
75H92N4O2Zr2, M = 1263.97, monoclinic, space group P21/n (no. 14), a
C
= 17.940(4), b = 18.075(4), c = 21.577(4) Å, b = 105.70(3)°, V =
6736(2) Å3, Z = 4, m = 0.36 mm21, T = 2138 °C, 21557 independent
reflections, Rint = 0.1058, 10074 observations, 752 parameters, R1
0.0637, wR2
=
= 0.1561, GOF =
0.929. For (Pri4Xan)Zr(CH2Ph)2:
C43H54N4OZr, M = 734.12, monoclinic, space group P21/c (no. 14), a =
10.887(2), b = 16.325(3), c = 21.427(4) Å, b = 96.47(3)°, V = 3784.1(13)
Å3, Z = 2, m = 0.33 mm21, T = 2138 °C, 12039 independent reflections,
Rint = 0.0554, 8425 observations, 452 parameters, R1 = 0.0455, wR2
=
0.1176, GOF = 1.008. Refinements were performed (SHELXTL-Plus
V5.0) on F2.
CCDC reference numbers 170218–170220. See http://www.rsc.org/
suppdata/cc/b1/b106972a/ for crystallographic data in CIF or other
electronic format.
1 For selected reviews, see: F. Fache, E. Schulz, M. L. Tommasino and M.
Lemaire, Chem. Rev., 2000, 100, 2159; A. Togni and L. M. Vananzi,
Angew. Chem., Int. Ed. Engl., 1994, 33, 497; M. J. Bartos, S. W. Gordon-
Wylie, B. G. Fox, L. J. Wright, S. T. Weintraub, K. E. Kauffmann, E.
Münck, K. L. Kostka, E. S. Uffelman, C. E. F. Rickard, K. R. Noon and
T. J. Collins, Coord. Chem. Rev., 1998, 174, 361; G. J. P. Britovsek, V.
C. Gibson and D. F. Wass, Angew. Chem., Int. Ed., 1999, 38, 428.
2 For metalloenzymes, see: F. A. Cotton and G. Wilkinson, Advanced
Inorganic Chemistry, Wiley Interscience, New York, 1988, 5th edn., ch.
30.
3 For selected synthetic examples and reviews, see: G. J. Rowlands,
Tetrahedron, 2001, 57, 1865; N. Wheatley and P. Kalck, Chem. Rev.,
1999, 99, 3379; Catalysis by Di- and Polynuclear Metal Cluster
Complexes, ed. R. D. Adams and F. A. Cotton, Wiley-VCH, Inc., New
York, 1998; R. G. Konsler, J. Karl and E. N. Jacobsen, J. Am. Chem. Soc.,
1998, 120, 10780; T. Ooi, M. Takahashi and K. Maruoka, Angew. Chem.,
Int. Ed., 1998, 37, 835; J. D. Wuest, Acc. Chem. Res., 1999, 32, 81; A.
Cottone III and M. J. Scott, Organometallics, 2000, 19, 5254; M. E.
Broussard, B. Juma, S. G. Train, W.-J. Peng, S. A. Laneman and G. G.
Stanley, Science, 1993, 260, 1784.
4 For other binucleating amidinates, see: J. R. Babcock, C. Incarvito, A. L.
Rheingold, J. C. Fettinger and L. R. Sita, Organometallics, 1999, 18,
5729; S. Appel, F. Weller and K. Dehnicke, Z. Anorg. Allg. Chem., 1990,
583, 7; C. Chen, L. H. Rees, A. R. Cowley and M. L. H. Green, J. Chem.
Soc., Dalton Trans., 2001, 1761; M. Ruben, D. Walther, R. Knake, H.
Görls and R. Beckert, Eur. J. Inorg. Chem., 2000, 5, 1055.
5 For related bis(phosphine) and bis(porphyrin) ligands, see: C. J. Chang,
Y. Q. Deng, A. F. Heyduk, C. K. Chang and D. G. Nocera, Inorg. Chem.,
2000, 39, 959; M. W. Haenel, D. Jakubik, E. Rothenberger and G.
Schroth, Chem. Ber., 1991, 124, 1705.
Fig. 3 Views of the molecular structure of (Pri4Xan)Zr(CH2Ph)2 drawn with
50% thermal ellipsoids. Pri groups omitted in (b) for clarity. Selected
distances (Å) and angles (°): Zr1–N1 2.269(2), Zr1–N2 2.247(2), Zr1–N3
2.288(2), Zr1–N4 2.308(2), Zr1–C30 2.280(2), Zr1–C37 2.284(2); C30–
Zr1–C37 88.87(8).
1
empirical formula [(Pri4Xan)Zr(CH2Ph)2]n. H NMR spectra
(C6D6) were also consistent with this formulation and featured
a single set of resonances for the benzyl ligands. Single crystal
X-ray diffraction studies revealed the product to be mono-
nuclear, with a single Zr atom bridging the two amidinates of
the (Pri4Xan) ligand (Fig. 3). Geometry at the Zr center closely
resembles that observed for Group 4 dialkyl derivatives of N4-
macrocycles.8 Interestingly, there appears to be significant ring
strain as a result of this binding mode. As shown in Fig. 3(b), the
amidinates are bent towards Zr giving angles of 114.1(2) and
115.2(2)° for C7–C8–C13 and C23–C21–C17, respectively.
Additionally, the Zr atom is coordinated ca. 1.2 Å out of each
NCN amidinate plane, and the Zr–N bond lengths suggest
stronger coordination to one of the amidinates. Asymmetric
binding, however, is not observed by 1H NMR spectroscopy (in
C6D6), and it is unclear whether this is due to fluxionality or a
lack of asymmetry.
6 D. Wood, G. P. A. Yap and D. S. Richeson, Inorg. Chem., 1999, 38,
5788.
We thank Professor Clark R. Landis for helpful discussions
and the University of Colorado for funding.
7 B. Qian, W. J. Scanlon, M. R. Smith and D. H. Morty, Organometallics,
1999, 18, 1693; R. F. Jordan, R. E. Lapointe, C. S. Bajgur, S. F. Echols
and R. Willett, J. Am. Chem. Soc., 1987, 109, 4111; S. L. Latesky, A. K.
McMullen, G. P. Niccolai, I. P. Rothwell and J. C. Huffman,
Organometallics, 1985, 4, 902.
Notes and references
† All compounds analyzed satisfactorily. Selected data for (Pri4DBF)H2: 1H
NMR (d6-acetone): d 8.12 (d, J 7.7 Hz, 2H), 7.42 (t, J 7.5 Hz, 2H), 7.33 (d,
J 7.3 Hz, 2H), 4.20 (br, 2H), 3.10 (br, 2H), 2.86 (br, 2H), 1.22 (br, 12H),
0.98 (br, 12H). For (Pri4Xan)H2: 1H NMR (d6-acetone): d 7.53 (d, J 7.7 Hz,
2H), 7.11 (t, J 7.5 Hz, 2H), 6.94 (d, J 7.1 Hz, 2H), 4.13 (br, 2H), 2.85 (m,
br, 4H), 1.63 (s, 6H), 1.36–0.80 (br, 24H). For (Pri4DBF)Zr2Bn6: 1H NMR
8 D. G. Black, D. C. Swenson, R. F. Jordan and R. D. Rogers,
Organometallics, 1995, 14, 3539; M. J. Scott and S. J. Lippard, Inorg.
Chim. Acta, 1997, 263, 287; H. Brand and J. Arnold, Coord. Chem. Rev.,
1995, 140, 137; L. Giannini, E. Solari, S. De Angelis, T. R. Ward, C.
Floriani, A. Chiesi-Villa and C. Rizzoli, J. Am. Chem. Soc., 1995, 117,
5801.
Chem. Commun., 2001, 2144–2145
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