Synthesis, structure, and reactivity of alkylzinc complexes stabilized with 1,1,3,3-tetramethylguanidine

Article abstract of DOI:10.1021/om700501g

The synthesis and structural characterization of several novel alkylzinc guanidinate, alkoxide, and aryloxide complexes are reported. 1,1,3,3-Tetramethylguanidine, H-TMG, was successfully reacted with (Et) ₂Zn in a 4:3 and a 1:1 ratio to yield

Full text of DOI:10.1021/om700501g

6320
Organometallics 2007, 26, 6320-6328
Synthesis, Structure, and Reactivity of Alkylzinc Complexes
Stabilized with 1,1,3,3-Tetramethylguanidine
Scott D. Bunge,* Jacob M. Lance, and Jeffrey A. Bertke
Department of Chemistry, Kent State UniVersity, Kent, Ohio 44242-0001
ReceiVed May 21, 2007
The synthesis and structural characterization of several novel alkylzinc guanidinate, alkoxide, and
aryloxide complexes are reported. 1,1,3,3-Tetramethylguanidine, H-TMG, was successfully reacted with
(Et)2Zn in a 4:3 and a 1:1 ratio to yield the corresponding linear [Zn3(µ-TMG)4(Et)2] (1) and cyclic
[Zn(µ-TMG)(Et)]3 (2) complexes. Further investigations have shown 2 to react with alcohols, HOR, to
form complexes with the general formula [Zn(Et)(OR)(H-TMG)]n, where OR ) OCH2C(CH3)3 {ONep,
n ) 2 (3)}, OC6H3(CMe3)2-2,6 {DBP, n ) 1 (4)}, and OC6H2But2-2,6-Me-4 {DBP-4-Me, n ) 1 (5)}.
The addition of (Et)2Zn to a solution of 4 yielded the unusual [Zn2(µ-TMG)(µ-DBP)(Et)2] (6). Complex
5 readily reacts with the alcohol, HOR′, yielding dinuclear compounds with the formula [Zn(µ-OR′)-
(DBP-4-Me)(H-TMG)]2, where OR′ ) OCH3 (OMe, 7) and OCH2CH3 (OEt, 8). Additionally, compound
4 was reacted with EtOH, yielding a complex with the formula [Zn(µ-OEt)(DBP)(H-TMG)]2 (9). The
utility of compound 8 for use as a catalyst in the ring-opening polymerization of rac-lactide was additionally
investigated. Compounds 1-9 were characterized by single-crystal X-ray diffraction. The bulk powders
for all complexes were found to be in agreement with the crystal structures based on elemental analyses,
1
FT-IR spectroscopy, and H and 13C NMR studies. The polymer was characterized by multi-nuclear
NMR spectroscopy and MALDI-TOF mass spectrometry.
additional Lewis bases, Zn alkoxides and aryloxides have been
found to form ill-defined polymetallic species.8 The ability of
H-TMG to support the isolation of well-defined systems is
therefore also assessed in this study.
Introduction
Alkylzinc (RZn) reagents have found extensive use in
catalysis, organic synthesis, and also in materials science.
Designing a co-ligand for RZn complexes to facilitate discrete
stoichiometric and catalytic reactivity has traditionally involved
a delicate interplay between the steric and electronic components
of the auxiliary ligand and the coordinative saturation of the
metal center.1-3 A general strategy to achieving this balance
would be the use of a conveniently obtained “versatile” ligand,
capable of facilitating coordinative saturation while hindering
entropically driven disassociation. One potential ligand possibly
capable of achieving this interplay is the imine of tetramethy-
lurea, the “super base” 1,1,3,3-tetramethylguanidine (H-TMG).
The synthesis of 1-9 was accomplished through examining
derivations of one of the oldest known class of organometallic
reactions: ethane elimination via reaction of diethyl zinc with
various ratios of guanidine and alcohol.9 The reaction of 4 and
3 equiv of H-TMG with 3 equiv of diethyl zinc, (Et)2Zn, was
performed. Products isolated from each reaction were found to
be the novel trinuclear [Zn3(µ-TMG)4(Et)2] (1) and [EtZn(µ-
TMG)]3 (2), respectively (Scheme 1). The general reactivity of
2 was further investigated. 2 reacts with HOR to form complexes
with the general formula [Zn(Et)(OR)(H-TMG)]n, where OR
For the past 20 years, the use of H-TMG has been sporadic
in the field of inorganic chemistry.4 This deficiency is surprising
because H-TMG belongs to one of the strongest and most
versatile classes of organic bases known.5 For comparison, they
are several orders of magnitude more basic than tertiary amines.6
Also, in addition to the strength, this enhanced basicity can be
“tuned” due to the excellent distribution of charge over the
resonance-stabilized guanidine structure.7 In this Article, we
present our effort to outline the stoichiometric reactivity of
TMG-ligated and H-TMG-ligated ethylzinc systems with alco-
hols of varied steric bulk. Historically, in the absence of
(4) Kuhn, N.; Fawzi, R.; Steimann, M.; Wiethoff, J. Z. Anorg. Allg. Chem.
1997, 623, 554. Schneider, W.; Bauer, A.; Schier, A.; Schmidbaur, H. Chem.
Ber. 1997, 130, 1417. De Vries, N.; Costello, C. E.; Jones, A. G.; Davison,
A. Inorg. Chem. 1990, 29, 1348. Pattison, I.; Wade, K.; Wyatt, B. K. J.
Chem. Soc. A 1968, 837. Bailey, P. J.; Pace, S. Coord. Chem. ReV. 2001,
214, 91. Edwards, A. J.; Paver, M. A.; Raithby, P. R.; Rennie, M. A.;
Russell, C. A.; Wright, D. S. J. Chem. Soc., Dalton Trans. 1995, 1587.
Stalke, D.; Paver, M. A.; Wright, D. S. Angew. Chem., Int. Ed. Engl. 1993,
32, 428. Davies, M. K.; Raithby, P. R.; Rennie, M. A.; Steiner, A.; Wright,
D. S. J. Chem. Soc., Dalton Trans. 1995, 2707. Alvarez, C. S.; Boss, S. R.;
Burley, J. C.; Humphry, S. M.; Layfield, R. A.; Kowenicki, R. A.; McPartlin,
M.; Rawson, J. M.; Wheatley, A. E. H.; Wood, P. T.; Wright, D. S. Dalton
Trans. 2004, 3481.
(5) Smith, P. A. S. The Organic Chemistry of Open-Chain Nitrogen
Compounds, Vol. I; Benjamin: New York, 1965. Patai, S., Ed. The Chemistry
of Functional Groups: the Chemistry of Amidines and Imidates; Wiley:
Chichester, 1975.
(6) Kovacevic, B.; Maksic, Z. B. Org. Lett. 2001, 3, 1523. Kovacevic,
B.; Maksic, Z. B. Chem.-Eur. J. 2002, 8, 1694. Novak, I.; Wei, X.; Chin,
W. S. J. Phys. Chem. A 2001, 105, 1783.
* To whom correspondence should be addressed. Phone: (330) 672-
9445. Fax: (330) 672-3816. E-mail: sbunge@kent.edu.
(1) Parkin, G. Chem. ReV. 2004, 104, 699. Cheng, M.; Moore, D. R.;
Reczek, J. J.; Chamberlain, B. M.; Lobkovsky, E. B.; Coates, G. W. J. Am.
Chem. Soc. 2001, 123, 8738. Darensbourg, D. J.; Holtcamp, M. W.; Struck,
G. E.; Zimmer, M. S.; Niezgoda, S. A.; Rainey, P.; Robertson, J. B.; Draper,
J. D.; Reibenspies, J. H. J. Am. Chem. Soc. 1999, 121, 107.
(2) Chisholm, M. H.; Gallucci, J.; Phomphrai, K. Inorg. Chem. 2002,
41, 2785.
(7) Coles, M. P. Dalton Trans. 2006, 985.
(8) Caulton, K. G.; Hubertpfalzgraf, L. G. Chem. ReV. 1990, 90, 969.
(9) Menard, D. F.; Aston, J. G. J. Am. Chem. Soc. 1934, 56, 1601.
Gilman, H.; Brown, G. E.; Webb, F. J.; Spatz, S. M. J. Am. Chem. Soc.
1940, 62, 977. Seyferth, D. Organometallics 2001, 20, 2940.
(3) Jensen, T. R.; Breyfogle, L. E.; Hillmyer, M. A.; Tolman, W. B.
Chem. Commun. 2004, 2504.
10.1021/om700501g CCC: $37.00 © 2007 American Chemical Society
Publication on Web 11/07/2007

Alkylzinc Complexes Stabilized with H-TMG
Organometallics, Vol. 26, No. 25, 2007 6321
Scheme 1. Synthesis of 1 and 2
Figure 1. Thermal ellipsoid plot of 1. Ellipsoids are drawn at the
30% level. H atoms have been omitted for clarity.
CH2CH3), 0.62 (q, 4.0H, CH2CH3). 13C{1H} NMR (100.5 MHz,
toluene-d8): δ ) 163.8 (NdC(N(CH3)2)2), 39.9 (NdC(N(CH3)2)),
14.0 (CH2CH3), 4.5 (CH2CH3). FT-IR (KBr, cm-1): 3058 (m), 2953
(s), 1546 (s), 1500 (m), 1457 (m), 1411 (s), 1372 (m), 1246 (s),
1196 (m), 1132 (s), 1101 (m), 1062 (m), 1033 (m), 987 (w), 959
(w), 920 (w), 901 (w), 880 (s), 859 (m), 816 (m), 787 (s), 754 (s),
662 (m) 622 (m), 569 (m), 530 (m), 467 (w), 443 (w).
[Zn(µ-TMG)(Et)]3 (2). H-TMG (0.72 g, 6.3 mmol) and (Et)2Zn
(4.4 g, 6.3 mmol) were used. Yield 85% (1.1 g, 1.79 mmol). Mp
98 °C. Anal. Calcd for C21H51N9Zn3: C, 40.30; H, 8.21; N, 20.14.
Found: C, 39.97; H, 8.11; N, 20.75. 1H NMR (400 MHz, toluene-
d8): δ ) 2.69 (s, 36.0H, 4NdC(N(CH3)2)2), 1.54 (t, 9.0H, CH2CH3),
0.47 (q, 6.0H, CH2CH3). 13C NMR (100.5 MHz, toluene-d8): δ )
160.9 (NdC(N(CH3)2)), 40.4 (NdC(N(CH3)2)), 14.1 (CH2CH3), 5.0
(CH2CH3). FT-IR (KBr, cm-1): 2928 (s), 2881 (s), 2800 (m), 1573
(s), 1538 (s), 1491 (m), 1462 (m), 1416 (m), 1402 (m), 1347 (s),
1249 (w), 1230 (w), 1139 (w), 1116 (s), 1058 (m), 1017 (m), 949
(w), 918 (w), 767 (w), 756 (m), 633 (w), 597 (w), 556 (m), 498
(w), 463 (w).
[Zn(Et)(H-TMG))(µ-ONEP)]2 (3). H-ONep (0.28 g 3.1 mmol)
and 2 (1.9 g 3.1 mmol) in hexanes were used. A solution of H-ONep
in hexanes (5 mL) was added dropwise to a stirring solution of 2
in hexanes. The solution was stirred for 15 min and then allowed
to sit and evaporate. The compound was subsequently recrystallized
from a concentrated hexanes solution at -37 °C to obtain
satisfactory elemental analysis. Yield 88% (0.82 g, 2.7 mmol). Mp
76 °C. Anal. Calcd for C24H58N6O2Zn2: C, 48.57; H, 9.85; N, 14.16.
Found: C, 48.01; H, 10.21; N, 14.48. 1H NMR (400 MHz, toluene-
d8): δ ) 5.17 (s, 2H, 2HNdCN(CH3)3), 3.72 (s, 4.0H, 2OCH2C-
(CH3)3), 2.65 (s, 18.0H, 2OCH2C(CH3)3), 2.33 (s, 24.0H, 2H-Nd
C(N(CH3)2)2), 1.77 (t, 6.0H, 2CH2CH3), 0.54 (q, 4.0H, 2CH2CH3).
13C NMR (100.5 MHz, toluene-d8): δ ) 167.8 (HNdC(N(CH3)2)),
78.6 (OCH2C(CH3)3), 39.9 (NdC(N(CH3)2)), 38.9 (CH2CH3), 35.0
(OCH2C(CH3)3), 27.9 (OCH2C(CH3)3), 15.3 (CH2CH3). FT-IR
(KBr, cm-1): 3349 (m), 2947 (s), 2803 (s), 2688 (m), 2639 (m),
1592 (s), 1536 (s), 1459 (s), 1402 (s), 1355 (m), 1281 (m), 1220
(m), 1116 (s), 1085 (s), 1021 (s), 947 (w), 902 (m), 773 (m), 749
(m), 593 (m), 488 (m), 425 (m).
) OCH2C(CH3)3 {ONep, n ) 2 (3)}, OC6H3(CMe3)2-2,6 {DBP,
n ) 1 (4)}, and OC6H2But2-2,6-Me-4 {DBP-4-Me, n ) 1 (5)}.
The addition of (Et)2Zn to a solution of 4 yielded [Zn2(µ-TMG)-
(µ-DBP)(Et)2] (6). 5 readily reacts with HOR′, yielding dinuclear
compounds: [Zn(µ-OR′)(DBP-4-Me)(H-TMG)]2, where OR′ )
OCH3 (OMe, 7) and OCH2CH3 (OEt, 8). Furthermore, 4 was
reacted with EtOH, yielding a complex with the formula [Zn-
(µ-OEt)(DBP)(H-TMG)]2 (9). The structures of these complexes
are shown in Figures 1-7, respectively. In addition, a prelimi-
nary investigation utilizing 8 as a catalyst in the ring-opening
polymerization of rac-lactide is also reported.
Experimental Section
All compounds were handled with rigorous exclusion of air and
water using standard glovebox techniques. All anhydrous solvents
were stored under argon and used as received in sure-seal bottles.
(Et)2Zn (1.0 M in hexanes), H-TMG, H-ONEP, H-DBP, MeOH,
EtOH, and rac-lactide were used as received from commercial
suppliers. FT-IR data were obtained on a Bruker Tensor 27
Instrument using KBr pellets under an atmosphere of flowing
nitrogen. Melting points were determined on samples sealed in a
glass tube under an atmosphere of argon using an Electrothermal
Mel-Temp apparatus and are uncorrected. Elemental analysis was
performed on a Perkin-Elmer 2400 Series 2 CHN-S/O Elemental
Analyzer.
Synthesis of 1 and 2. 1 and 2 were synthesized by adding the
appropriate amount of (Et)2Zn (1.0 M) dropwise to a solution of
H-TMG dissolved in hexanes (10 mL). After being stirred for 30
min, the reaction mixture was allowed to sit in the glovebox until
the solvent evaporated and crystals formed. The complexes were
subsequently recrystallized from a concentrated hexanes solution
at -37 °C to obtain satisfactory elemental analysis.
[Zn(Et)(DBP)(H-TMG)] (4). H-DBP (1.3 g, 6.3 mmol), Zn-
(Et)2 (4.4 g, 6.3 mmol), H-TMG (0.72 g, 6.3 mmol), and 5 mL of
hexane were used. Yield 87% (2.2 g, 5.4 mmol). Mp 113 °C. Anal.
Calcd for C21H38N3OZn: C, 60.79; H, 9.47; N, 10.13. Found: C,
1
60.92; H, 9.70; N, 9.65. H NMR (400 MHz, toluene-d8): δ )
[Zn3(µ-TMG)4(Et)2] (1). H-TMG (0.96 g, 8.3 mmol) and (Et)2Zn
(4.4 g, 6.3 mmol) were used. Yield 97% (1.4 g, 2.0 mmol). Mp
(dec) 140 °C. Anal. Calcd for C24H58N12Zn3: C, 40.54; H, 8.22;
N, 23.64. Found: C, 40.41; H, 8.21; N, 23.91. 1H NMR (400 MHz,
toluene-d8): δ ) 2.71 (s, 48.0H, NdC(N(CH3)2)2), 1.64 (t, 6.0H,
7.35 (m, 1H, OC6H3(C(CH3)3)2), 6.68 (d, 2H, OC6H3(C(CH3)3)2)),
4.38 (s, 1H, HNdCN(CH3)3), 2.28 (s, 6H, HNdC(N(CH3)2)), 1.85
(s, 6H, HNdC(N(CH3)2)), 1.71 (s, 18H, OC6H3(C(CH3)3)2-2,6), 1.57
(t, 3H, CH2CH3), 0.63 (q, 2H, CH2CH3). 13C NMR (100.5 MHz,
toluene-d8): δ ) 167.1 (HNdC(N(CH3)2)), 138.6, 124.6 113.5,

6322 Organometallics, Vol. 26, No. 25, 2007
Bunge et al.
115.2 (OC6H3(C(CH3)3)2-2,6), 38.7 (HNdC(N(CH3)2)), 37.6 (OC6H3-
(C(CH3)3)2-2,6), 31.1 (OC6H3(C(CH3)3)2-2,6), 12.6 (CH2CH3), 1.0
(CH2CH3). FT-IR (KBr, cm-1): 3362 (w), 2963 (s), 2905 (m), 1577
(m), 1542 (m), 1446 (s), 1412 (m), 1259 (s), 1110 (s), 864 (s), 802
(s), 754 (m), 702 (m), 687 (m), 663 (m), 625 (m), 505 (w).
(OC6H2(C(CH3)3)2-2,6-(CH3)-4), 21.3 (OC6H2(C(CH3)3)2-2,6-(CH3)-
4), 21.0 (OCH2CH3). FT-IR (KBr, cm-1): 3362 (m), 3337 (m),
2957 (s), 1577 (s), 1541 (s), 1457 (m), 417 (s), 1382 (m), 1262 (s)
1219 (w), 1128 (s), 1100 (s), 1056 (s), 889 (m), 859 (m), 803 (s),
762 (w), 723 (w), 603 (w), 557 (w), 517 (m), 441 (m).
[Zn(µ-OEt)(DBP)(H-TMG)]2 (9). H-DBP (2.2 g, 3.2 mmol),
Zn(Et)2 (2.2 g, 3.2 mmol), H-TMG (0.36 g, 3.2 mmol), and EtOH
(0.14 g, 3.2 mmol) were used. Yield 99% (1.3 g, 1.6 mmol). Mp
165 °C. Anal. Calcd for C42H78N6O4Zn2: C, 58.53; H, 9.12; N,
9.75. Found: C, 58.57; H, 9.25; N, 9.13. 1H NMR (400 MHz, THF-
d8): δ ) 7.07 (m, 2H, OC6H3(C(CH3)3)2-2,6), 6.92 (d, 2H, OC6H3-
(C(CH3)3)2-2,6), 4.26 (s, 1H, HNdCN(CH3)3), 3.91 (q, 2H,
OCH2CH3), 2.92 (s, 6H, HNdC(N(CH3)2)), 2.50 (s, 6H, HNd
C(N(CH3)2)), 1.44 (s, 18H, OC6H3(C(CH3)3)2-2,6), 1.04 (t, 2H,
OCH2CH3). 13C NMR (100.5 MHz, THF-d8): δ ) 167.5 (HNd
C(N(CH3)2)), 139.7, 138.2, 125.5, 125.1 (OC6H3(C(CH3)3)2-2,6),
61.4 (OCH2CH3), 39.9 (HNdC(N(CH3)2)), 37.8 (OC6H3(C(CH3)3)2-
2,6), 32.1 (OC6H3(C(CH3)3)2-2,6), 21.6 (OCH2CH3). FT-IR (KBr,
cm-1): 3365 (m), 2949 (s), 2803 (w), 1572 (s), 1541 (s), 1458
(m), 1404 (s), 1379 (s), 1348 (m), 1324 (w), 1306 (w), 1268 (s),
1202 (m), 1128 (m), 1099 (s), 1059 (m), 1032 (s), 892 (m), 861
(m), 817 (m), 742 (s), 717 (m), 643 (w), 599 (w), 558 (w), 491
(m), 439 (m).
[Zn(Et)(DBP-4-Me)(H-TMG)] (5). H-DBP-4Me (0.69 g, 3.2
mmol), Zn(Et)2 (2.2 g, 3.2 mmol), H-TMG (0.36 g, 3.2 mmol),
and 5 mL of hexanes were used. Yield 97% (1.3 g, 3.1 mmol). Mp
115 °C. Anal. Calcd for C22H41N3OZn: C, 61.60; H, 9.63; N, 9.80.
1
Found: C, 61.14; H, 9.23; N, 9.80. H NMR (400 MHz, toluene-
d8): δ ) 7.14 (s, 2H, OC6H2(C(CH3)3)2-2,6-(CH3)-4), 4.46 (s, 1H,
HNdCN(CH3)3), 2.40 (s, 3H, OC6H2(C(CH3)3)2-2,6-(CH3)-4), 2.29
(s, 6H, HNdC(N(CH3)2)), 1.87 (s, 6H, HNdC(N(CH3)2)), 1.71 (s,
18H, OC6H2(C(CH3)3)2-2,6-(CH3)-4), 1.55 (t, 3H, CH2CH3), 0.61
(q, 2H, CH2CH3). 13C NMR (100.5 MHz, toluene-d8): δ ) 167.7
(HNdC(N(CH3)2)), 165.3, 138.9, 126.0, 121.4 (OC6H2(C(CH3)3)2-
2,6-(CH3)-4), 39.4, 38.3 (HNdC(N(CH3)2)), 35.9 (OC6H2(C(CH3)3)2-
2,6-(CH3)-4), 32.1 (OC6H2(C(CH3)3)2-2,6-(CH3)-4), 22.2 (OC6H2-
(C(CH3)3)2-2,6-(CH3)-4), 13.4 (CH2CH3), 1.7 (CH2CH3). FT-IR
(KBr, cm-1): 3179 (w), 2924 (s), 2852 (s), 2796 (m), 1583 (s),
1547 (m), 1417 (m), 1400 (m), 1339 (m), 1250 (m), 1226 (m),
1141 (m), 1106 (s), 1057 (m), 1012 (m), 948 (w), 917 (m), 759
(m), 627 (m), 599 (m), 553 (m), 499 (w), 457 (w).
Polymerization Procedure. Under an argon atmosphere, com-
plex 8 was reacted with rac-lactide (LA) ([LA]:[cat] ) 200) in a
Schlenk flask charged with dichloromethane. The reaction was
stirred for 24 h and then quenched with methanol. A white polymer
resulted after removing the solvent. The resulting polymer was
analyzed by 1H and 13C NMR spectroscopy and MALDI-TOF mass
spectrometry (Figures 9 and S3).
[(Zn2(µ-TMG)(µ-DBP)(Et)2] (6). H-DBP (1.29 g, 6.3 mmol),
Zn(Et)2 (8.8 g, 12.5 mmol), and H-TMG (0.72 g, 6.3 mmol) were
used. Yield 73% (2.2 g, 4.6 mmol). Mp 104 °C. Anal. Calcd for
C23H43N3OZn2: C, 54.34; H, 8.53; N, 8.27. Found: C, 54.57; H,
8.96; N, 8.30. 1H NMR (400 MHz, toluene-d8): δ ) 7.30 (m, 1H,
OC6H3(C(CH3)3)2), 6.79 (d, 2H, OC6H3(C(CH3)3)2-2,6), 2.46 (s,
12H, NdC(N(CH3)2)2), 1.63 (s, 18H, OC6H3(C(CH3)3)2-2,6), 1.25
(t, 3H, CH2CH3), 0.51 (q, 2H, CH2CH3). 13C NMR (100.5 MHz,
toluene-d8): δ ) 160.7 (NdC(N(CH3)2)), 139.8, 126.1, 118.5, 115.9
(OC6H3(C(CH3)3)2), 39.6 (NdC(N(CH3)2)), 35.7 (OC6H3(C(CH3)3)2-
2,6), 33.2 (OC6H3(C(CH3)3)2-2,6), 12.8 (CH2CH3), 3.5 (CH2CH3).
FT-IR (KBr, cm-1): 2963 (s), 2905 (m), 1944 (w), 1574 (w), 1446
(w), 1413 (m), 1261 (s), 1020 (s), 864 (m), 798 (m), 702 (m), 503
(w), 405 (w).
X-ray Crystal Structure Information. X-ray crystallography
was performed by mounting each crystal onto a thin glass fiber
from a pool of Fluorolube and immediately placing it under a 100
K N2 stream, on a Bruker AXS diffractometer. The radiation used
was graphite monochromatized Mo KR radiation (λ ) 0.7107 Å).
The lattice parameters were optimized from a least-squares calcula-
tion on carefully centered reflections. Lattice determination, data
collection, structure refinement, scaling, and data reduction were
carried out using APEX2 version 1.0-27 software package.
Each structure was solved using direct methods. This procedure
yielded the Zn atoms, along with a number of the C, N, and O
atoms. Subsequent Fourier synthesis yielded the remaining atom
positions. The hydrogen atoms were fixed in positions of ideal
geometry and refined within the XSHELL software. These idealized
hydrogen atoms had their isotropic temperature factors fixed at 1.2
or 1.5 times the equivalent isotropic U of the C atoms to which
they were bonded. The final refinement of each compound included
anisotropic thermal parameters on all non-hydrogen atoms. Data
collection parameters are listed in Tables 1-3. Interatomic distances
and angles are listed in Tables 4-6. Additional information
concerning the data collection and final structural solutions of
compounds 1-9 can be found in the Supporting Information. Any
variations from standard structural solution associated with the
representative compounds are discussed below.
[Zn(µ-OCH3)(DBP-4Me)(H-TMG)]2*2(THF) (7). H-DBP-4Me
(0.69 g, 3.1 mmol), Zn(Et)2 (2.2 g, 3.1 mmol), H-TMG (0.36 g,
3.1 mmol), and MeOH (0.10 g, 3.1 mmol) were used. Yield 75%
(1.3 g, 1.2 mmol). Mp 188 °C. Anal. Calcd for C50H94N6O6Zn2:
C, 59.69; H, 9.42; N, 8.35. Found: C, 59.27; H, 9.22; N, 8.38. 1H
NMR (400 MHz, THF-d8): δ ) 7.16 (s, 2H, OC6H2(C(CH3)3)2-
2,6-(CH3)-4), 3.99 (s, 1H, HNdCN(CH3)3), 3.80 (s, 3H, OCH3),
2.70 (s, 6H, HNdC(N(CH3)2)), 2.51 (s, 3H, OC6H2(C(CH3)3)2-2,6-
(CH3)-4), 1.84 (s, 6H, HNdC(N(CH3)2)), 1.74 (s, 18H, OC6H2-
(C(CH3)3)2-2,6-(CH3)-4). 13C NMR (100.5 MHz, THF-d8): δ )
168.0 (HNdC(N(CH3)2)), 165.6, 139.3, 125.3, 120.7 (OC6H2-
(C(CH3)3)2-2,6-(CH3)-4), 54.6 (OCH3), 39.5 (HNdC(N(CH3)2)),
35.6 (OC6H2(C(CH3)3)2-2,6-(CH3)-4), 31.3 (OC6H2(C(CH3)3)2-2,6-
(CH3)-4), 21.2 (OC6H2(C(CH3)3)2-2,6-(CH3)-4). FT-IR (KBr, cm-1):
3335 (s), 2950 (s), 2802 (m), 1581 (s), 1415 (s), 1309 (m), 1255
(s), 1129 (s), 1063 (s), 935 (w), 894 (m), 856 (w), 801 (s), 749 (s),
599 (w), 523 (w), 458 (s), 406 (w).
[Zn(µ-OEt)(DBP-4-Me)(H-TMG)]2 (8). H-DBP-4-Me (0.69 g,
3.1 mmol), Zn(Et)2 (2.2 g, 3.1 mmol), H-TMG (0.36 g, 3.1 mmol),
and EtOH (0.14 g, 3.1 mmol) were used. Yield 89% (1.2 g, 1.4
mmol). Mp 156 °C. Anal. Calcd for C44H82N6O4Zn2: C, 59.38; H,
Results and Discussion
General Considerations. The syntheses of complexes 1-9
were performed under an argon atmosphere with the objective
of outlining the reactivity of H-TMG with Et2Zn. All compounds
were isolated as colorless crystalline compounds in good yield.
1-9 were moderately to slightly soluble in hexanes and readily
soluble in THF.
Synthesis. The syntheses of 1 and 2 are shown in Scheme 1.
In a hexane solution, the straightforward reaction of 3 or 4 equiv
of 1,1,3,3-tetramethylguanidine (H-TMG) with 3 equiv of
diethyl zinc results in the immediate evolution of CH3CH3.
1
9.29; N, 9.44. Found: C, 59.00; H, 9.31; N, 9.41. H NMR (400
MHz, toluene-d8): δ ) 7.18 (s, 2H, OC6H2(C(CH3)3)2-2,6-(CH3)-
4), 4.38 (s, 1H, HNdCN(CH3)3), 4.25 (q, 2H, OCH2CH3), 2.66 (s,
6H, HNdC(N(CH3)2)), 2.40 (s, 3H, OC6H2(C(CH3)3)2-2,6-(CH3)-
4), 1.93 (s, 6H, HNdC(N(CH3)2)), 1.73 (s, 18H, OC6H2(C(CH3)3)2-
2,6-(CH3)-4), 1.28 (t, 3H, OCH2CH3). 13C NMR (100.5 MHz,
toluene-d8): δ ) 166.7 (HNdC(N(CH3)2)), 164.6, 138.3, 125.3,
120.4 (OC6H2(C(CH3)3)2-2,6-(CH3)-4), 60.5 (OCH2CH3), 38.8
(HNdC(N(CH3)2)), 35.4 (OC6H2(C(CH3)3)2-2,6-(CH3)-4), 31.5

Alkylzinc Complexes Stabilized with H-TMG
Organometallics, Vol. 26, No. 25, 2007 6323
Table 1. Data Collection Parameters for 1-3
Table 4. Selected Interatomic Distances (Å) and Angles
(deg) for 1 and 2
compound
Complex 1
1
2
3
Zn(1)-N(4)
Zn(1)-N(7)
Zn(2)-N(1)
Zn(2)-C(21)
Zn(3)-N(10)
N(1)-C(1)
1.995(6)
2.024(5)
1.964(5)
1.964(8)
1.978(5)
1.285(8)
1.373(8)
Zn(1)-N(10)
Zn(1)-N(1)
Zn(2)-N(7)
Zn(3)-N(4)
Zn(3)-C(23)
N(2)-C(1)
2.021(5)
2.034(5)
1.983(5)
1.976(5)
1.981(10)
1.394(8)
chemical formula
formula weight
temp (K)
C24H58N12Zn3 C21H51N9Zn3
710.93
100(2)
monoclinic
P2(1)/n
13.5902(13)
14.3845(13)
18.5827(17)
C24H58N6O2Zn2
593.50
625.82
100(2)
triclinic
P-1
10.083(2)
11.379(2)
13.732(3)
92.734(4)
99.931(4)
99.860(4)
1524.0(5)
2
100(2)
space group
triclinic
P-1
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
9.5136(17)
9.9491(18)
10.1261(18)
117.446(3)
98.305(3)
94.292(3)
830.7(3)
1
N(3)-C(1)
N(4)-Zn(1)-N(7)
122.9(2)
N(7)-Zn(1)-N(10) 122.7(2)
N(4)-Zn(1)-N(10)
N(4)-Zn(1)-N(1)
N(10)-Zn(1)-N(1)
N(1)-Zn(2)-N(7)
N(4)-Zn(3)-N(10)
N(10)-Zn(3)-C(23) 133.2(4)
Zn(3)-N(4)-Zn(1) 91.5(2)
Zn(3)-N(10)-Zn(1) 90.7(2)
85.9(2)
121.9(2)
122.1(2)
89.0(2)
87.6(2)
N(7)-Zn(1)-N(1)
85.9(2)
101.762(2)
N(1)-Zn(2)-C(21) 135.4(3)
C(21)-Zn(2)-N(7) 132.7(3)
N(4)-Zn(3)-C(23) 137.7(4)
Zn(2)-N(1)-Zn(1) 90.5(2)
Zn(2)-N(7)-Zn(1) 90.2(2)
3556.4(6)
4
D
calcd (Mg/m3)
1.328
2.039
1.364
2.366
1.186
1.470
µ (Mo KR) (mm-1
)
R1a (%) (all data)
5.59 (13.84)
4.51 (5.89)
11.88 (12.95) 8.92 (10.26)
3.23 (4.66)
Complex 2
wR2b (%) (all data) 12.16 (15.88)
Zn(1)-N(7)
Zn(1)-N(1)
Zn(2)-N(4)
Zn(3)-N(4)
Zn(3)-N(1)
N(1)-C(7)
1.960(2)
Zn(1)-C(1)
Zn(2)-C(3)
Zn(2)-N(7)
Zn(3)-C(5)
N(3)-C(7)
N(2)-C(7)
1.983(3)
1.984(3)
1.998(2)
1.987(3)
1.390(4)
1.414(4)
2
-
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo| × 100. b wR2 ) [∑w(Fo
Fc )2/
2.021(2)
1.984(2)
1.978(2)
2.001(2)
1.278(4)
2
∑(w|Fo| )2]1/2 × 100.
Table 2. Data Collection Parameters for 4-6
compound
N(7)-Zn(1)-C(1)
C(1)-Zn(1)-N(1)
C(3)-Zn(2)-N(7)
N(4)-Zn(3)-C(5)
C(5)-Zn(3)-N(1)
133.46(14) N(7)-Zn(1)-N(1)
124.89(14) C(3)-Zn(2)-N(4)
120.13(13) N(4)-Zn(2)-N(7)
130.76(12) N(4)-Zn(3)-N(1)
131.40(12) Zn(3)-N(1)-Zn(1)
99.62(10)
134.74(13)
105.09(10)
97.60(9)
101.43(10)
101.81(11)
4
5
6
chemical formula
formula weight
temp (K)
C21H39N3OZn C22H41N3OZn C23H43N3OZn2
414.92
100(2)
monoclinic
P2(1)/n
11.186(2)
13.866(3)
15.376(3)
428.95
100(2)
508.34
188(2)
Zn(3)-N(4)-Zn(2) 104.72(11) Zn(1)-N(7)-Zn(2)
space group
monoclinic
P2(1)/c
9.2716(5)
12.0066(8)
21.7463(14)
monoclinic
P2(1)
10.4473(9)
17.4330(14)
14.5079(12)
Table 5. Selected Interatomic Distances (Å) and Angles
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
(deg) for 3-6
Complex 3
Zn(1)-O(1)
Zn(1)-O(1A)
N(1)-C(6)
N(3)-C(6)
2.0152(19) Zn(1)-N(1)
2.074(2)
1.996(3)
1.361(4)
99.819(4)
94.9850(10)
90.339(2)
2.0200(19) Zn(1)-C(11)
1.299(4)
1.376(4)
N(2)-C(6)
2349.9(8)
4
2411.6(3)
4
2642.2(4)
4
D
calcd (Mg/m3)
1.173
1.058
1.181
1.033
1.278
1.832
C(11)-Zn(1)-O(1)
C(11)-Zn(1)-N(1)
O(1A)-Zn(1)-N(1)
122.18(11) O(1)-Zn(1)-O(1A ) 84.75(8)
128.00(12) O(1)-Zn(1)-N(1)
87.76(9) C(1)-O(1)-Zn(1)
µ (Mo KR) (mm-1
)
97.06(9)
121.01(17)
R1a (%) (all data)
5.67 (12.41)
4.97 (7.52)
7.54 (7.88)
4.18 (7.10)
7.96 (8.38)
wR2b (%) (all data) 13.82 (18.81)
Zn(1)-O(1)-Zn(1A) 95.25(7)
a R1 ) ∑||Fo| - |Fc||/∑|Fo| × 100. b wR2 ) [∑w(Fo
Fc )2/
2
2
-
Complex 4
2
∑(w|Fo| )2]1/2 × 100.
Zn(1)-O(1)
Zn(1)-N(1)
N(2)-C(15)
1.900(3)
1.957(5)
1.349(6)
Zn(1)-C(20)
N(1)-C(15)
N(3)-C(15)
1.943(6)
1.327(7)
1.348(7)
Table 3. Data Collection Parameters for 7-9
compound
O(1)-Zn(1)-C(20)
O(1)-Zn(1)-N(1)
135.7(2)
90.04(17)
C(20)-Zn(1)-N(1)
C(1)-O(1)-Zn(1)
134.2(3)
141.6(3)
7
8
9
chemical formula C50H94N6O6Zn2 C44H82N6O4Zn2 C42H78N6O4Zn2
Complex 5
formula weight
temp (K)
1006.05
100(2)
89.90
100(2)
861.84
100(2)
Zn(1)-O(1)
Zn(1)-N(1)
N(2)-C(16)
1.9075(19) Zn(1)-C(21)
1.965(4)
1.311(3)
1.362(4)
1.961(2)
1.359(3)
N(1)-C(16)
N(3)-C(16)
space group
triclinic
P-1
monoclinic
P2(1)/c
triclinic
P-1
O(1)-Zn(1)-C(21)
C(21)-Zn(1)-N(1)
132.37(13) O(1)-Zn(1)-N(1)
139.26(13) C(1)-O(1)-Zn(1)
88.24(9)
128.49(17)
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
9.2885(13)
9.6888(13)
16.675(2)
84.828(2)
80.272(2)
66.688(2)
1357.9(3)
1
10.2512(17)
16.578(3)
14.609(3)
9.5000(14)
9.805(2)
14.350(2)
71.257(6)
79.161(17)
64.983(9)
1144.9(3)
1
1.250
1.092
4.08 (6.43)
10.47 (12.17)
Complex 6
Zn(1)-C(20)
Zn(1)-O(1)
Zn(2)-C(22)
N(1)-C(15)
N(3)-C(15)
1.929(6)
2.038(3)
1.958(9)
1.313(7)
1.367(7)
Zn(1)-N(1)
Zn(2)-N(1)
Zn(2)-O(1)
N(2)-C(15)
1.926(5)
1.909(4)
2.079(4)
1.361(7)
91.008(4)
2482.2(7)
2
1.191
1.009
4.15 (7.85)
10.90 (13.82)
D
calcd (Mg/m3)
1.230
0.933
3.71 (5.42)
µ (Mo KR) (mm-1
)
C(20)-Zn(1)-N(1)
N(1)-Zn(1)-O(1)
N(1)-Zn(2)-O(1)
Zn(2)-N(1)-Zn(1)
146.6(2)
83.65(17)
82.97(16)
101.2(2)
C(20)-Zn(1)-O(1)
N(1)-Zn(2)-C(22)
C(22)-Zn(2)-O(1)
Zn(1)-O(1)-Zn(2)
128.8(2)
147.2(4)
129.8(4)
92.05(14)
R1a (%) (all data)
wR2b (%) (all data) 10.35 (12.32)
2
-
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo| × 100. b wR2 ) [∑w(Fo
Fc )2/
2
∑(w|Fo| )2]1/2 × 100.
solvents. Retention of the zinc-bonded ethyl group for 1 and 2
is implied by the observation of a triplet (δ ) 1.64 for 1 and
1.54 for 2) and a quartet (δ ) 0.62 for 1 and 0.47 for 2) by 1H
NMR spectroscopy. FT-IR spectroscopy was utilized to confirm
Reducing the solutions to dryness provides colorless crystals
of [Zn3(µ-TMG)4(Et)2] (1) and [Zn(µ-TMG)(Et)]3 (2), respec-
tively. Both complexes are readily soluble in hydrocarbon

6324 Organometallics, Vol. 26, No. 25, 2007
Bunge et al.
Table 6. Selected Interatomic Distances (Å) and Angles
until each reaction went clear. Notably, the DBP-4Me deriva-
tives (7 and 8) demonstrated increased solubility in hexanes
and toluene versus compound 9. Crystals of 7-9 were grown
by allowing their respective reaction mixtures to cool and slowly
evaporate. 1H and 13C NMR spectroscopy were utilized to
confirm the presence of OR′ (OR′ ) OMe or OEt) for 7-9.
FT-IR spectroscopy on 7-9 confirmed absorption bands cor-
responding to υ(N-H) and υ(CdN) stretching modes around
3100 and 1580 cm-1, respectively.
(deg) for 7-9
Complex 7
Zn(1)-O(1)
Zn(1)-O(2)
O(2)-Zn(1A)
1.9316(18) Zn(1)-O(2A)
1.9743(19) Zn(1)-N(1)
1.9795(19)
1.9795(19)
1.985(2)
O(1)-Zn(1)-O(2)
O(2)-Zn(1)-O(2A)
O(2)-Zn(1)-N(1)
111.44(8)
82.80(8)
123.76(9)
O(1)-Zn(1)-O(2A) 122.01(8)
O(1)-Zn(1)-N(1) 104.73(9)
O(2A)-Zn(1)-N(1) 112.37(9)
Zn(1)-O(2)-Zn(1A) 97.20(8)
Structural Descriptions. [Zn3(µ-TMG)4(Et)2] (1). The
thermal ellipsoid plot of compound 1 is illustrated in Figure 1.
1 is perhaps best compared to the previously reported Zn
phosphoraneiminato complex, [Zn3(µ-NdPMe3)4{N(SiMe3)2}2].10
The structure consists of a nonlinear array of three metal atoms
(Zn‚‚‚Zn‚‚‚Zn ) 164.17°) that are bridged by four TMG ligands.
The terminal zincs are further coordinated to ethyl groups. The
central metal has a distorted tetrahedral geometry, and the two
terminal zincs have a trigonal-planar coordination. The Zn-N
distances in 1 average 2.011 Å for the central zinc and 1.975 Å
for the outer zincs. For the terminal zincs, the average Zn-C
distance is 1.972 Å. The CdN imino donors range from 1.264
to 1.284 Å and are typical for a carbon-nitrogen double bond.7
The other two C-N distances range from 1.372 to 1.412(4) Å,
and the interactions of these N atoms with zincs are negligible
(the shortest Zn‚‚‚N distance is 3.406 Å).
[Zn(µ-TMG)(Et)]3 (2). The X-ray crystallographic study of
2 shows it to exist as a cyclic trinuclear complex in the solid
state (Figure 2). This is comparable to the cyclic [Zn{µ-
P(Cyclohexyl)2}(Et)]3 published by Wright and co-workers.11
The complex has a chair-shaped Zn3N3 hexanuclear core in with
Zn-N bonds range from 1.959 to 2.021 Å. The Zn centers have
almost identical trigonal planar geometries (∑ of the three angles
about Zn average 359.9°). The ring angles at Zn and N are
100.8° (average) for N-Zn-N and 102.7° (average) for Zn-
N-Zn. Additionally, all of the zincs are coordinated to ethyl
groups with an average Zn-C distance of 1.984 Å.
[Zn(Et)(Η-ΤΜG))(µ-ONEP)]2 (3). Figure 3 shows the
thermal ellipsoid plot of 3. The structure of 3, similar to the
previously described pyridine adduct [Zn(Et)(py))(µ-ONEP)]2,12
proved to be a centro-symmetric dinuclear complex wherein
each zinc atom is tetrahedrally bound by two bridging neopen-
toxide ligands, one terminal ethyl group, and one H-TMG
ligand. The Zn-O distances in 3 average 2.018 Å, and the
Zn-C distances are 1.995 Å. The Zn-N distances of 2.074 Å
are consistent with assigning the additional coordination of
H-TMG to zinc.
[Zn(Et)(DBP)(H-TMG)] (4) and [Zn(Et)(DBP-4-Me)(H-
TMG)] (5). The structures of 4 and 5 are illustrated in the
thermal ellipsoid plot of 4 (Figure 4). Both 4 and 5 are examples
of neutral monomeric three-coordinate zinc complexes and
perhaps best compared to the three-coordinate [Zn(Me)(DBP)-
{Me2NC(NiPr)2}] described by Coles and co-workers.13 Each
trigonal planar zinc metal center was determined to be coordi-
nated to one aryloxide, one ethyl group, and one H-TMG. The
bond lengths (Zn-O, Zn-C, and Zn-N) are very similar for 4
and 5. A shorter Zn-O diatance (1.900 Å) is observed for 4
and 5 as compared to [Zn(Me)(DBP){Me2NC(NiPr)2}] (1.951
Å).13
Complex 8
Zn(1)-O(1)
Zn(1)-O(2A)
N(1)-C(18)
N(3)-C(18)
1.926(2)
Zn(1)-O(2)
Zn(1)-N(1)
N(2)-C(18)
1.967(2)
1.988(3)
1.355(5)
1.975(2)
1.309(4)
1.363(5)
O(1)-Zn(1)-O(2)
O(2)-Zn(1)-O(2A)
O(2)-Zn(1)-N(1)
122.16(10) O(1)-Zn(1)-O(2A) 113.90(10)
81.79(11) O(1)-Zn(1)-N(1) 101.37(12)
117.91(11) O(2A)-Zn(1)-N(1) 120.49(13)
Zn(1)-O(2)-Zn(1A) 98.21(11)
C(1)-O(1)-Zn(1)
132.9(2)
C(16)-O(2)-Zn(1)
129.4(2)
Complex 9
Zn(1)-O(1)
Zn(1)-O(3)
O(3)-Zn(1A)
N(2)-C(15)
1.935(2)
1.972(2)
1.971(2)
1.356(4)
Zn(1)-O(3A)
Zn(1)-N(1)
N(1)-C(15)
N(3)-C(15)
1.971(2)
1.990(3)
1.314(4)
1.357(4)
O(1)-Zn(1)-O(3A)
O(1)-Zn(1)-O(3)
O(3A)-Zn(1)-N(1)
112.12(9)
123.05(10) O(1)-Zn(1)-N(1)
124.86(11) O(3)-Zn(1)-N(1)
O(3A)-Zn(1)-O(3) 81.58(9)
101.75(10)
114.63(10)
136.24(19)
Zn(1A)-O(3)-Zn(1) 98.42(9)
C(20)-O(3)-Zn(1) 127.29(19)
C(1)-O(1)-Zn(1)
the υ(CdN) of absorption bands around 1550 cm-1 correspond-
ing to the Nimino coordinated to zinc.
The reactions shown in Scheme 2 were followed for the
synthesis of the novel family of “Zn(Et)(OR)(H-TMG)” com-
pounds (3-5). To synthesize 3-5, complex 2 was dissolved in
hexane and allowed to react with the respective H-OR for an
hour. For each derivative, a precipitate formed that dissolved
upon heating the solution. Colorless crystals were grown by
allowing the reaction mixtures to cool and slowly evaporate or
by cooling the reaction mixture (-37 °C). FT-IR spectroscopy
on 3-5 confirmed absorption bands corresponding to υ(N-H)
and υ(CdN) stretching modes around 3100 and 1580 cm-1
,
respectively. 3-5 show no OH peaks around 3000 cm-1, which
is consistent with full exchange. For the Zn-O interactions,
there should be a stretch present between 800 and 400 cm-1
.
For each sample, a stretch around 755 cm-1 was observed,
which is tentatively assigned to the Zn-O. H and 13C NMR
1
spectroscopy were once again utilized to confirm retention of
the zinc-bonded ethyl groups for 3-5.
The reaction of Et2Zn with complex 4 in hexanes results in
the rapid evolution of CH3CH3 (see Scheme 2). Evaporation of
the solvent resulted in the isolation of [(Zn2(µ-TMG)(µ-DBP)-
(Et)2] (6) as colorless crystals in good yield. 1H NMR
spectroscopy for 6 confirmed the existence of the zinc-bonded
ethyl group by the observation of a triplet (δ ) 1.64) and a
quartet (δ ) 0.62). FT-IR spectroscopy on 6 confirms absorption
bands corresponding to υ(CdN) band at 1546 cm-1. In addition,
the spectrum for 6 exhibits no N-H peaks around 3100 cm-1
which is consistent with full exchange. For the Zn-O interac-
tions, a stretch at 754 cm-1 was observed, which is tentatively
assigned to the Zn-O interaction.
The syntheses of 7-9 are shown in Scheme 3. For each
reaction, “Zn(Et)(OR)(H-TMG)” was dissolved in hexanes and
allowed to react with H-OR′ (OR′ ) OMe or OEt). CH3CH3
evolved, and a precipitate formed that would not dissolve even
upon heating. THF was added, and the reaction was heated again
,
(10) Krieger, M.; Gould, R. O.; Neumuller, B.; Harms, K.; Dehnicke,
K. Z. Anorg. Allg. Chem. 1998, 624, 1434.
(11) Edwards, A. J.; Paver, M. A.; Raithby, P. R.; Russell, C. A.; Wright,
D. S. Organometallics 1993, 12, 4687.
(12) Boyle, T. J.; Bunge, S. D.; Andrews, N. L.; Matzen, L. E.; Sieg,
K.; Rodriguez, M. A.; Headley, T. J. Chem. Mater. 2004, 16, 3279.
(13) Coles, M. P.; Hitchcock, P. B. Eur. J. Inorg. Chem. 2004, 2662.

Alkylzinc Complexes Stabilized with H-TMG
Organometallics, Vol. 26, No. 25, 2007 6325
Scheme 2. Synthesis of 3-6
Scheme 3. Synthesis of 7-9
[(Zn2(µ-TMG)(µ-DBP)(Et)2] (6). Compound 6 crystallizes
in the monoclinic space group P2(1) with two crystallographicly
inequivalent, but nearly identical, dinuclear zinc complexes per
unit cell. The structure of 6 is depicted in Figure 5. Each trigonal
planar zinc atom is bridged to the adjacent zinc via one TMG
[Zn-N 1.918 Å (average)] and one aryloxide ligand [Zn-O
2.058 Å (average)]. One ethyl group is additionally coordinated
to each metal center [Zn-C 1.944 Å (average)].
[Zn(µ-OCH3)(DBP-4Me)(H-TMG)]2*2(THF) (7), [Zn(µ-
OEt)(DBP-4-Me)(H-TMG)]2 (8), and [Zn(µ-OEt)(DBP)(H-
TMG)]2 (9). The structures of compounds 7, 8, and 9 may be
illustrated by the thermal ellipsoid plots of 7 and 8 shown in
Figures 6 and 7, respectively. All three complexes have a planar
Zn2O2 core where each zinc atom is tetrahedrally coordinated
by two bridging alkoxide groups, one terminal aryloxide, and
one H-TMG ligand. The terminal aryloxide ligands of 7, 8, and
9 notably exhibit a small Zn-O-Ar angle, demonstrating the
absence of π-donation from the aryl ring, a feature typically
found present in early transition metal aryloxide compounds.14
PGSE Measurements. To examine whether 7-9 exist
primarily in solution as a monomer or dinuclear complex, pulsed
gradient spin-echo (PGSE) NMR measurements were per-
formed.15 The PGSE method has been successful at assessing
effective hydrodynamic volumes of inorganic complexes in
solution through determination of the diffusion coefficient, a
number related to the translational motion of a compound in
solution.16 Because of the similarity in size of 5 to the expected
(14) Bradley, D. C.; Mehrotra, R. C.; Rothwell, I. P.; Singh, A. Alkoxo
and Aryloxo DeriVatiVes of Metals; 2001.
(15) Price, W. S. Concepts Magn. Reson. 1997, 9, 299.

6326 Organometallics, Vol. 26, No. 25, 2007
Bunge et al.
Figure 4. Thermal ellipsoid plot of 4. Ellipsoids are drawn at the
30% level. H atoms have been omitted for clarity.
Figure 2. Thermal ellipsoid plot of 2. Ellipsoids are drawn at the
30% level. H atoms have been omitted for clarity.
Figure 5. Thermal ellipsoid plot of 6. Ellipsoids are drawn at the
30% level. H atoms have been omitted for clarity. There are two
crystallographically independent, but chemically equivalent, mol-
ecules present in the asymmetric unit. One molecule is shown.
Figure 3. Thermal ellipsoid plot of 3. Ellipsoids are drawn at the
30% level. H atoms have been omitted for clarity.
monomeric form of 8, PGSE experiments on d8-toluene solu-
tions of 8 and 5 were performed. The results are displayed in
Figure 8, plotted as ln(I/I0) (where I ) observed spin-echo
intensity and I0 ) intensity in the absence of gradients), versus
a function with the square of the gradient strength over a
constant diffusion time. Each data point represents an average
of the values obtained for the different resonances in the NMR
spectrum of each compound at varying gradient strengths. The
slopes of the lines in Figure 8 are equal to the diffusion
coefficients (D) according to eq 1. The absolute value of the
slope decreases with increasing effective hydrodynamic radii
according to the Stokes-Einstein eq 2. The experimentally
determined values of D and r were calculated and are listed in
Table 7. From these data, the effective hydrodynamic volume
for 8 was determined to be greater than that for 5. For
comparison, we also estimated values of the molecular radii
(r′) for 5 and 8 from their X-ray crystal structures by measuring
Figure 6. Thermal ellipsoid plot of 7. Ellipsoids are drawn at the
30% level. H atoms have been omitted for clarity.
the lengths between centers of distant hydrogen atoms on the
molecular periphery. All of the complexes were considered as
prolate ellipsoids (Table 7), and r′ was determined by using eq
3. The values of r (PGSE) and r′ (X-ray) for 5 were found to
closely agree. In contrast, for 8, the value of r determined by
the PGSE measurement is 28% smaller than the radius estimated
(16) Geldbach, T. J.; Pregosin, P. S.; Albinati, A.; Rominger, F.
Organometallics 2001, 20, 1932. Pichota, A.; Pregosin, P. S.; Valentini,
M.; Worle, M.; Seebach, D. Angew. Chem., Int. Ed. 2000, 39, 153.
Mukherjee, R.; Dougan, B. A.; Fry, F. H.; Bunge, S. D.; Ziegler, C. J.;
Brasch, N. E. Inorg. Chem. 2007, 46, 1575.

Alkylzinc Complexes Stabilized with H-TMG
Organometallics, Vol. 26, No. 25, 2007 6327
Figure 9. 1H NMR spectra (400 MHz, CDCl3) of PLA methine
resonances with selective decoupling of PLA methyl resonances.
Figure 7. Thermal ellipsoid plot of 8. Ellipsoids are drawn at the
30% level. H atoms have been omitted for clarity.
suggestion, one can further estimate an upper bound for the
radius expected for the monomeric 8, r′ ) 5.1 Å, by adding
approximately one-half of the distance of an O-Et bond (0.7
Å) to the r′ value for 5 (4.4 Å). This estimated r′ was then used
to calculate D′ ) 7.1 × 10-10 m2 s-1 (assuming a spherical
shape for the molecule). Both the estimated r′ and D′ values
for the supposed monomeric form closely agree with those that
were experimentally determined for 8. Therefore, we can
conclude 8 has a monomeric structure in solution.
rac-Lactide Polymerization. Both organic and inorganic
catalysts have been reported for the ring-opening polymerization
of lactide (LA).3,13,17,18 In fact, a number of elegant syntheses
utilizing Zn alkoxides supported by tris(pyrazolyl)hydroborate
or â-diketiminate ligands have been performed and were found
to produce quite active and, in some cases, stereoselective
polymerization catalysts.2,18,19 The structures of 7-9 resemble
recently reported active Zn catalysts.20 Inspired by this resem-
blance, we explored the capability of 8 to polymerize rac-LA.
It should be noted that there are at least two possible problems
with this approach: disassociation of both H-TMG and/or
aryloxide ligands resulting in the formation of ill-defined
catalytic species. However, preliminary results have demon-
strated neither of these issues to be a problem. In fact, Zn
complexes coordinated with H-TMG are crystallized from THF
with no THF found coordinated to the metal center. In all of
our studies, there has been no evidence that H-TMG disassoci-
ates from the metal in favor of coordinating another Lewis base.
As for the potential reactivity of the aryloxide ligand, its
disassociation is tempered by its interaction with the H-TMG
moiety, the steric bulk located on the ring, and also the
utilization of a highly reactive alkoxide substituent (OR′) on
the metal center.
8 was reacted with rac-lactide (LA) ([LA]:[cat] ) 200) in
dichloromethane for 1 day and then quenched with methanol.
A white polymer resulted after removing the solvent. The
resultant polymer was examined by 1H NMR spectroscopy (Mn
) 24.7 kg mol-1), and the tacticity was investigated by
examination of the PLA methine resonances with selective
decoupling of PLA methyl resonances (Figure 9). The percent
of heterotactic tetrads (isi and sis) indicates a low degree of
heterotacticity. The resultant polymer was further examined with
MALDI-TOF mass spectrometry (Figure S3). The spectrum
Figure 8. Plot of ln(I/I0) versus γ2δ2G2[∆ - (δ/3)] × 1010 (m-2
s) from PGSE experiments for 5 (9) and 8 (2).
Table 7. PGSE Experimental Results As Compared to
Estimated Data from X-ray Crystal Structuresa
Db
D′′b
complex (×1010 m2 s-1
)
rc (Å) ad (Å) bd (Å) r′e (Å) (×1010 m2 s-1
)
5
8
9.0(2)
7.5(1)
4.0(3) 6.3(7) 3.6(1) 4.3(6)
4.8(4) 8.9(7) 5.9(2) 6.7(9)
8.3(4)
5.3(5)
a Estimated errors are indicated in parentheses, with terms defined in
the text. b From slopes of the lines in Figure 12, eq 1, and eq 2. c The errors
are estimated to be ∼5% primarily on the basis of the accuracy of η used
in the calculation. d Estimated values from the X-ray crystal structure by
measuring the lengths between the centers of distant hydrogen atoms on
the periphery corresponding to each arbitrary axis and by calculating the
minimum ellipsoid into which the molecule fits. e Calculated using eq 3.
Equation 3: r′ ) (ab2)1/3. Equation 2: D ) kT/(6πηr). Equation 1: ln(I/I0)
) -γ2δ2G2(∆ - δ/3)D (G ) gradient strength, γ ) gyromagnetic ratio, δ
) length of gradient pulse, ∆ ) delay between gradient midpoints, r )
effective hydrodynamic radius, and η ) viscosity).
from the X-ray crystal structure (r′) of the dinuclear form. These
data strongly suggest that 8 does not retain its dinuclear form
in solution. We also calculated diffusion coefficients (D′) from
the values of a, b, and r′, which required the use of a variant of
eq 2 appropriate for prolate ellipsoids. As expected, the
agreement between the PGSE measured (D) and predicted (D′)
diffusion coefficients is excellent for 5 (consistent with the
agreement of r and r′), but D′ for 8 is much less than the
measured value, again consistent with the form of 8 in solution
being significantly smaller than the dinuclear structure adopted
in the crystal. On this basis, therefore, one can suggest that 8 is
predominantly a monomer in d8-toluene. To confirm this
(17) Myers, M.; Connor, E. F.; Glauser, T.; Mock, A.; Nyce, G.; Hedrick,
J. L. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 844.
(18) C0hisholm, M. H.; Eilerts, N. W.; Huffman, J. C.; Iyer, S. S.; Pacold,
M.; Phomphrai, K. J. Am. Chem. Soc. 2000, 122, 11845.
(19) Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.;
Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 3229.
(20) Williams, C. K.; Breyfogle, L. E.; Choi, S. K.; Nam, W.; Young,
V. G.; Hillmyer, M. A.; Tolman, W. B. J. Am. Chem. Soc. 2003, 125, 11350.

6328 Organometallics, Vol. 26, No. 25, 2007
Bunge et al.
indicates polymer with little cyclic ether formation, and also
little transesterification.
Tolman and co-workers.20 These reported Zn complexes dem-
onstrated potential for use as polymerization catalysts. We
therefore demonstrated that 8 exists as a monomer in solution
and is an active catalyst for the ring-opening polymerization of
rac-LA to PLA.
Discussion
The synthesis of 1-9 was accomplished via the straightfor-
ward reaction of diethyl zinc with various ratios of guanidine
and alcohol. The volatility of the ethane side product typically
serves to drive the forward reaction and thus facilitate nearly
quantitative yield. Following this approach resulted in the clean
formation of the guanidinate complexes 1 and 2. The formation
of relatively strong Zn-N and C-H bonds, at the expense of
breaking the N-H and a relatively weak Zn-C bond, ensures
favorable reaction energies.
Further reaction of 2 with an additional equivalent of alcohol
resulted in the isolation of the hetero-ligated complexes 3-5.
For this series of reactions, protonation of the guanidinate ligand
and the formation of a Zn-O bond were energetically preferred
versus breaking an additional Zn-C bond, forming a C-H bond
and liberating ethane. This stepwise reactivity is without
precedent in organo-zinc chemistry.
Summary and Conclusion
We have successfully synthesized and characterized a series
of novel guanidinate- and guanidine-stabilized Zn compounds.
The complexes were characterized through use of single-crystal
X-ray diffraction, elemental analysis, FT-IR, and H and 13C
1
NMR. The reactions took place stoichiometrically producing
mononuclear, dinuclear, or trinuclear compounds. Compound
8 was found to be a monomer in solution and successfully
catalyzed the ring-opening polymerization of rac-lactide into
poly(LA). Control of polymer stereochemistry and molecular
weight could possibly be achieved by altering the substituents
in the guanidine and aryloxide ligands. Additional work in this
area is currently under investigation.
The reactivity of H-TMG coordinated to Zn was examined
via the reaction of 4 with a stoichiometric equivalent of diethyl
zinc. Deprotonation of the coordinated H-TMG and the libera-
tion of ethane resulted in the formation of the dinuclear complex
6. In 6, each trigonal planar Zn is connected to the adjacent Zn
via bridging TMG and aryloxide ligands. The Zn atoms are
further coordinated to terminal ethyl substituents. This remark-
ably “simple” low-coordinate Zn2NO ring is without structural
precedent. It is noteworthy that preliminary studies of the
reactivity of 6 with alcohols were performed and the dinuclear
Zn core was found to not be retained. Instead, a mixture of
Zn-containing products is formed.
The synthesis of 7, 8, and 9 was performed by reacting 4
and 5 with either methanol or ethanol. Ethane is readily
liberated, and the corresponding dinuclear Zn alkoxide is isolated
in good yield. For complexes 7-9, the tetrahedral Zn atom is
coordinated to a terminal aryloxide, H-TMG, and is connected
to the other Zn via two bridging alkoxide ligands. These
complexes resemble the recently reported dinuclear Zn alkoxide
complexes with general formula [Zn(µ-OR)(L)]2 reported by
Acknowledgment. We thank Dr. A. Khitrin (KSU) for
assistance with the PGSE measurements, Dr. M. Gangoda
(KSU) for assistance with the MALDI-TOF measurements, and
J. A. Ocana (KSU) for assistance with elemental analysis. We
thank Kent State University for financial support of this work.
Supporting Information Available: Thermal ellipsoid plots
for 5 (Figure S1) and 9 (Figure S2). MALDI-TOF mass spectrum
of PLA (Figure S3). This material is available free of charge via
the Internet at http://pubs.acs.org. Crystallographic data (excluding
structure factors) for the structures have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publica-
tion nos. CCDC 647796 for 1, CCDC 647797 for 2, CCDC 647798
for 3, CCDC 647799 for 4, CCDC 647800 for 5, CCDC 647801
for 6, CCDC 647802 for 7, CCDC 647803 for 8, and CCDC 647804
for 9. Copies of the data can be obtained, free of charge, via http://
pubs.acs.org or on application to CCDC, 12 Union Road, Cam-
bridge CB2 1EZ, UK (fax, +44-(0)1223-336033; or e-mail,
deposit@ccdc.cam.ac.uk).
OM700501G